Mastodon API: Preloading and normalization optimizations

- Try to normalize the activity instead of object wherever possible
- Put the `user` key on non-home timelines as well so bookmarks and
thread mutes are preloaded there as well
- Skip trying to get the user when rendering mentions if the id ==
as:Public or user's follower collection
- Preload the object when getting replied to activities and do not crash
if it's not present

This almost solves the problem of Pleroma hammering the db with a lot
of queries when rendering timelines, the things left are
1. When rendering mentions and the user is not in cache, save it for
later and request all uncached users in one go
2. Somehow get rid of needing to get the latest follow activity to
detect the value of `requested` in a relationship. (create a database
view for user relationship and cache it maybe?)

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