Follow #29833 and add tests

Introduce a well-tested `svg.Normalize` function.
Make `RenderHTML` faster and more stable.

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>

Result of running `perl -p -i -e 's#interface\{\}#any#g' **/*` and `make fmt`.

Basically the same [as golang did](2580d0e08d).

The plan is that all built-in auth providers use inline SVG for more
flexibility in styling and to get the GitHub icon to follow
`currentcolor`. This only removes the `public/img/auth` directory and
adds the missing svgs to our svg build.

It should map the built-in providers to these SVGs and render them. If
the user has set a Icon URL, it should render that as an `img` tag
instead.

```
gitea-azure-ad
gitea-bitbucket
gitea-discord
gitea-dropbox
gitea-facebook
gitea-gitea
gitea-gitlab
gitea-google
gitea-mastodon
gitea-microsoftonline
gitea-nextcloud
gitea-twitter
gitea-yandex
octicon-mark-github
```

GitHub logo is now white again on dark theme:

<img width="431" alt="Screenshot 2023-06-12 at 21 45 34"
src="https://github.com/go-gitea/gitea/assets/115237/27a43504-d60a-4132-a502-336b25883e4d">

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

The idea is to use a Layered Asset File-system (modules/assetfs/layered.go)

For example: when there are 2 layers: "custom", "builtin", when access
to asset "my/page.tmpl", the Layered Asset File-system will first try to
use "custom" assets, if not found, then use "builtin" assets.

This approach will hugely simplify a lot of code, make them testable.

Other changes:

* Simplify the AssetsHandlerFunc code
* Simplify the `gitea embedded` sub-command code

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

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>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Also added more checks for the render function.

Co-authored-by: silverwind <me@silverwind.io>

Go 1.17 and later use modern `//go:build` constraints, the old `// +build:` constraints should be removed.

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* Add bundle download

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Fix fmt

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Fix build tags

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Download specific commit

Signed-off-by: jolheiser <john.olheiser@gmail.com>

Introduce 'make svg' which calls a node script that compiles svg files
to `public/img/svg`. These files are vendored to not create a dependency
on Node for the backend build.

On the frontend side, configure webpack using `raw-loader` so SVGs can
be imported as string.

Also moved our existing SVGs to web_src/svg for consistency.

Fixes: https://github.com/go-gitea/gitea/issues/11618