Remove endpoints for individual community/user inboxes

[Nutomic]

We dont use these inboxes at all, and the standard doesnt have any uniqueness requirement for inboxes. So we can get rid of them to simplify the code. And by storing only shared inboxes in a separate, deduplicated table, we store much less data and can reduce the database size.

TODO:

• This line doesnt compile, it would have to read the inbox url from db which requires context and async.
• inbox_url currently has a default value from generate_unique_changeme() which is mainly used for tests. Need to make it work for foreign key so tests work.
• Need to remove unused entries from inbox table (eg after a user is deleted from db).

[Nutomic]

Replaced by https://github.com/LemmyNet/lemmy/pull/5124, this approach is too complicated.