Backport #25171 by @silverwind
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: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
