Add more checks in inbox, plus some refactoring (#76)

Add more checks in inbox, plus some refactoring (#76)

Merge branch 'main' into more-inbox-permissions

Move check_community_ban() into helper function

Move slur check into helper functions

Move Claims::decode and site ban check into helper function

Note: this changes behaviour in that site ban is checked in more
places now. we could easily add a boolean parameter
check_for_site_ban to get the previous behaviour back

Rewrite user_inbox and community_inbox in the same way as shared_inbox

Add check against instance allowlist etc in shared_inbox

Co-authored-by: dessalines <dessalines@noreply.yerbamate.dev>
Co-authored-by: Felix Ableitner <me@nutomic.com>
Reviewed-on: https://yerbamate.dev/LemmyNet/lemmy/pulls/76