Fix #25473
Although there was `m.Post("/login/oauth/access_token", CorsHandler()...`,
it never really worked, because it still lacks the "OPTIONS" handler.
Replace #25892
Close #21942
Close #25464
Major changes:
1. Serve "robots.txt" and ".well-known/security.txt" in the "public"
custom path
* All files in "public/.well-known" can be served, just like
"public/assets"
3. Add a test for ".well-known/security.txt"
4. Simplify the "FileHandlerFunc" logic, now the paths are consistent so
the code can be simpler
5. Add CORS header for ".well-known" endpoints
6. Add logs to tell users they should move some of their legacy custom
public files
```
2023/07/19 13:00:37 cmd/web.go:178:serveInstalled() [E] Found legacy public asset "img" in CustomPath. Please move it to /work/gitea/custom/public/assets/img
2023/07/19 13:00:37 cmd/web.go:182:serveInstalled() [E] Found legacy public asset "robots.txt" in CustomPath. Please move it to /work/gitea/custom/public/robots.txt
```
This PR is not breaking.
---------
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>
`http.ServeFile/ServeContent` handles `If-xxx`, `Content-Length`,
`Range` and `Etag` correctly
After this PR, storage files (eg: avatar) could be responded with
correct Content-Length.
