Use inline SVG for built-in OAuth providers (#25171) (#25234)

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 <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>

Author: 3 people

modules/svg/svg.go

@@ -62,5 +62,5 @@ func RenderHTML(icon string, others ...interface{}) template.HTML {
 		}
 		return template.HTML(svgStr)
 	}
-	return template.HTML("")
+	return ""
 }