Use restricted sanitizer for repository description (#28141)

Use restricted sanitizer for repository description (#28141)

- Currently the repository description uses the same sanitizer as a
normal markdown document. This means that element such as heading and
images are allowed and can be abused.
- Create a minimal restricted sanitizer for the repository description,
which only allows what the postprocessor currently allows, which are
links and emojis.
- Added unit testing.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1202
- Resolves https://codeberg.org/Codeberg/Community/issues/1122

(cherry picked from commit 631c87cc2347f0036a75dcd21e24429bbca28207)

Co-authored-by: Gusted <postmaster@gusted.xyz>