Move `public/*` to `public/assets/*`

Some old PRs (like #15219) introduced inconsistent directory system.

For example: why the local directory "public" is accessed by
`http://site/assets`? How to serve the ".well-known" files properly in
the public directory?

For convention rules, the "public" directory is widely used for the
website's root directory. It shouldn't be an exception for Gitea.

So, this PR makes the things consistent:

* `http://site/assets/foo` means `{CustomPath}/public/assets/foo`.
* `{CustomPath}/public/.well-known` and `{CustomPath}/public/robots.txt`
can be used in the future.

This PR is also a prerequisite for a clear solution for:
* #21942
* #25892 
* discourse.gitea.io: [.well-known path serving custom files behind
proxy?](https://discourse.gitea.io/t/well-known-path-serving-custom-files-behind-proxy/5445/1)

This PR is breaking for users who have custom "public" files (CSS/JS).
After getting approvals, I will update the documents.

----

## ⚠️ BREAKING ⚠️

If you have files in your "custom/public/" folder, please move them to
"custom/public/assets/".

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Giteabot <teabot@gitea.io>

Fix #23409 

Developers could browse & preview the local SVG images files directly.

It still has clear output.

![image](https://user-images.githubusercontent.com/2114189/224317107-f4b26c76-e36a-4e80-9eee-d8dc2e16421f.png)


![image](https://user-images.githubusercontent.com/2114189/224317527-2d4ca131-978c-4933-b071-4bae483f06e1.png)

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <leon@kske.dev>

- Update all JS dependencies to latest version
- Disable two redundant eslint rules
- Adapt stylelint config to codebase
- Regenerate SVGs
- Make file editor spinner "reserve" height so page does not shift
- Tested katex, swagger, monaco

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>

* Added package store settings.

* Added models.

* Added generic package registry.

* Added tests.

* Added NuGet package registry.

* Moved service index to api file.

* Added NPM package registry.

* Added Maven package registry.

* Added PyPI package registry.

* Summary is deprecated.

* Changed npm name.

* Sanitize project url.

* Allow only scoped packages.

* Added user interface.

* Changed method name.

* Added missing migration file.

* Set page info.

* Added documentation.

* Added documentation links.

* Fixed wrong error message.

* Lint template files.

* Fixed merge errors.

* Fixed unit test storage path.

* Switch to json module.

* Added suggestions.

* Added package webhook.

* Add package api.

* Fixed swagger file.

* Fixed enum and comments.

* Fixed NuGet pagination.

* Print test names.

* Added api tests.

* Fixed access level.

* Fix User unmarshal.

* Added RubyGems package registry.

* Fix lint.

* Implemented io.Writer.

* Added support for sha256/sha512 checksum files.

* Improved maven-metadata.xml support.

* Added support for symbol package uploads.

* Added tests.

* Added overview docs.

* Added npm dependencies and keywords.

* Added no-packages information.

* Display file size.

* Display asset count.

* Fixed filter alignment.

* Added package icons.

* Formatted instructions.

* Allow anonymous package downloads.

* Fixed comments.

* Fixed postgres test.

* Moved file.

* Moved models to models/packages.

* Use correct error response format per client.

* Use simpler search form.

* Fixed IsProd.

* Restructured data model.

* Prevent empty filename.

* Fix swagger.

* Implemented user/org registry.

* Implemented UI.

* Use GetUserByIDCtx.

* Use table for dependencies.

* make svg

* Added support for unscoped npm packages.

* Add support for npm dist tags.

* Added tests for npm tags.

* Unlink packages if repository gets deleted.

* Prevent user/org delete if a packages exist.

* Use package unlink in repository service.

* Added support for composer packages.

* Restructured package docs.

* Added missing tests.

* Fixed generic content page.

* Fixed docs.

* Fixed swagger.

* Added missing type.

* Fixed ambiguous column.

* Organize content store by sha256 hash.

* Added admin package management.

* Added support for sorting.

* Add support for multiple identical versions/files.

* Added missing repository unlink.

* Added file properties.

* make fmt

* lint

* Added Conan package registry.

* Updated docs.

* Unify package names.

* Added swagger enum.

* Use longer TEXT column type.

* Removed version composite key.

* Merged package and container registry.

* Removed index.

* Use dedicated package router.

* Moved files to new location.

* Updated docs.

* Fixed JOIN order.

* Fixed GROUP BY statement.

* Fixed GROUP BY #2.

* Added symbol server support.

* Added more tests.

* Set NOT NULL.

* Added setting to disable package registries.

* Moved auth into service.

* refactor

* Use ctx everywhere.

* Added package cleanup task.

* Changed packages path.

* Added container registry.

* Refactoring

* Updated comparison.

* Fix swagger.

* Fixed table order.

* Use token auth for npm routes.

* Enabled ReverseProxy auth.

* Added packages link for orgs.

* Fixed anonymous org access.

* Enable copy button for setup instructions.

* Merge error

* Added suggestions.

* Fixed merge.

* Handle "generic".

* Added link for TODO.

* Added suggestions.

* Changed temporary buffer filename.

* Added suggestions.

* Apply suggestions from code review

Co-authored-by: Thomas Boerger <thomas@webhippie.de>

* Update docs/content/doc/packages/nuget.en-us.md

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Thomas Boerger <thomas@webhippie.de>