Allow all URL schemes in Markdown links by default (#24805)

- Closes #21146
- Closes #16721

## ⚠️ BREAKING ⚠️
This changes the default behavior to now create links for any URL scheme
when the user uses the markdown form for links (`[label](URL)`), this
doesn't affect the rendering of inline links. To opt-out set the
`markdown.CUSTOM_URL_SCHEMES` setting to a list of allowed schemes, all
other schemes (except `http` and `https`) won't be allowed.

# Before

![image](https://github.com/go-gitea/gitea/assets/20454870/35fa18ce-7dda-4995-b5b3-3f360f38296d)

# After

![image](https://github.com/go-gitea/gitea/assets/20454870/0922216b-0b35-4b77-9919-21a5c21dd5d0)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: Giteabot <teabot@gitea.io>

custom/conf/app.example.ini

@@ -1327,6 +1327,7 @@ ROUTER = console
 ;; Comma separated list of custom URL-Schemes that are allowed as links when rendering Markdown
 ;; for example git,magnet,ftp (more at https://en.wikipedia.org/wiki/List_of_URI_schemes)
 ;; URLs starting with http and https are always displayed, whatever is put in this entry.
+;; If this entry is empty, all URL schemes are allowed.
 ;CUSTOM_URL_SCHEMES =
 ;;
 ;; List of file extensions that should be rendered/edited as Markdown