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>

Diff without whitespace:
https://github.com/go-gitea/gitea/pull/24740/files?diff=unified&w=1

- Use SVGs for GitHub and GitLab oauth providers
- Replace section wrapping with a divider
- Rework icon rendering, increase size from 32px to 40px

Before:

<img width="853" alt="Screenshot 2023-05-15 at 21 54 23"
src="https://github.com/go-gitea/gitea/assets/115237/6ab5cfb4-46ff-469a-bd1f-06780d4a6a0b">

After (more providers):

<img width="849" alt="Screenshot 2023-05-15 at 21 51 21"
src="https://github.com/go-gitea/gitea/assets/115237/fa84f92f-98e0-4aed-9357-5d62ddd98195">

<img width="856" alt="Screenshot 2023-05-15 at 21 56 45"
src="https://github.com/go-gitea/gitea/assets/115237/d3edd7ed-dadd-4302-aca7-08f20adc220e">


Ref: https://codeberg.org/Codeberg/Community/issues/1023

---------

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 JS dependencies

- Update all JS dependencies
- For octicons, rename trashcan to trash
- For svgo, migrate to v2 api, output seems to have slightly changed but icons look the same
- For stylelint, update config, fix custom property duplicates
- For monaco, drop legacy Edge support
- For eslint, enable new rules, fix new issues
- For less-loader, remove deprecated import syntax

* update svgo usage in generate-images and rebuild logo.svg with it

* Add a migrat service type switch page

* Improve translations

* remove images

* Fix images

* remove extra create repo button on dashboard

* Follow reviewers' opinions

* Fix frontend lint

* Remove wrong submit file

* Fix tests

* Adjust the size of image

* Apply suggestions from code review

Co-authored-by: 赵智超 <1012112796@qq.com>

* Remove username and password from migration of github/gitlab

* Improve docs

* Improve interface docs

Co-authored-by: 赵智超 <1012112796@qq.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>