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](35fa18ce-7dda-4995-b5b3-3f360f38296d)

# After

![image](0922216b-0b35-4b77-9919-21a5c21dd5d0)

---------

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

docs/content/doc/administration/config-cheat-sheet.en-us.md

@@ -276,7 +276,7 @@ The following configuration set `Content-Type: application/vnd.android.package-a
   trailing whitespace to paragraphs is not necessary to force a line break.
 - `CUSTOM_URL_SCHEMES`: Use a comma separated list (ftp,git,svn) to indicate additional
   URL hyperlinks to be rendered in Markdown. URLs beginning in http and https are
-  always displayed
+  always displayed. If this entry is empty, all URL schemes are allowed
 - `FILE_EXTENSIONS`: **.md,.markdown,.mdown,.mkd,.livemd**: List of file extensions that should be rendered/edited as Markdown. Separate the extensions with a comma. To render files without any extension as markdown, just put a comma.
 - `ENABLE_MATH`: **true**: Enables detection of `\(...\)`, `\[...\]`, `$...$` and `$$...$$` blocks as math blocks.