User profile images and banner images => extract to table. Add `is_downloaded` column. Add test for double-downloading a user, shouldn't change the write-times on these files
- "Tweet is unavailable" tombstones are not always accurate. In user feed, they show as "not available" but the real reason is "deleted" etc; should update the tombstone type when fetching the conversation view
- tweet-to-stub: If a tweet is deleted after getting a non-stub version, should still show a tombstone (or a modified one at least)
- stub-to-tweet: If a user un-privates after getting a stub-version (or tweet becomes available for other reasons), stub versions should write the real content and "stub" status should be removed
- "Account owner limits who can view these tweets" tombstone: https://twitter.com/michaelmalice/status/1479540552081326085
TODO https://twitter.com/michaelmalice/status/1461031030278742020 => tombstone should say "account no longer exists" but it says "Tweet is unavailable"