Get rid of individual community/user inboxes

[Nutomic]

Requirements

• [X] Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support
• [X] Did you check to see if this issue already exists?
• [X] Is this only a feature request? Do not put multiple feature requests in one issue.
• [X] Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Is your proposal related to a problem?

Lemmy uses /inbox for federation, but also has endpoints like /c/memes/inbox or /u/nutomic/inbox. The latter were only added because other Activitypub platforms like Mastodon have them, but they dont really serve any purpose in Lemmy. Instead they add extra complexity and cause bugs like https://github.com/LemmyNet/lemmy/issues/3928.

The Activitypub standard doesnt require inbox urls to be unique so we can expect other platforms to support it. https://www.w3.org/TR/activitypub/#inbox https://www.w3.org/TR/activitypub/#shared-inbox-delivery

Describe the solution you'd like.

Get rid of shared_inbox_url columns for users and communities, instead set /inbox as inbox_url for them. Lemmy has a unique constraint on that column until 0.19.0 so this change breakes federation with earlier versions and should be implemented in 0.20 at the earliest. See https://github.com/LemmyNet/lemmy/pull/4138