add options to mix prune_objects to delete more things

Related issue: #3042

This is based on https://akkoma.dev/AkkomaGang/akkoma/pulls/350 and a few others. Commits:

• 7695010268d54bf7939fb1c2d5e33e7bce5b6989
• a7ec6e039cdc9ca5a0cc744fbd51511c5ede9f29
• 57eef6d76492e772f83acba2402d50ecb6a69f6b
• f49e9e6d4cb7cfdd0c2fbb2ea74c82a7ec7d13e4
• c1c962e1a80873a22cdc328fffb446f550892b10

This adds three options to mix prune_objects:

• --keep-threads: Don't prune posts when they are part of a thread where at least one post has seen local interaction
• --keep-non-public: Keep non-public posts like DM's and followers-only, even if they are remote.
• --prune-orphaned-activities: Prune orphaned activities (Like, Announce, etc.) related to deleted posts. Note: very slow.

With --keep-non-public --prune-orphaned-activities, my database shrunk by about 60%.

Breaking change: the current task does not remove non-public posts by default. With this PR, administrators will need to use the --keep-non-public flag for the same behavior.

Checklist

• Adding a changelog: In the changelog.d directory, create a file named <code>.<type>.