Upgrade to v2 API via Twift

[daneden]

Also a bunch of other enhancements! WIP!

To do

Accounts

• [ ] Add multiple account support
• [ ] Allow different colour themes for different accounts

Media

• [x] Fix media attachments
• [x] Allow alt text
• [x] Allow multiple photos
• [x] Allow gifs
• [x] Fix thumbnail rotation
• [ ] Fix drafts with media
  • I'm not sure there's a way to do this in any straightforward way; PHPicker doesn't make possible to restore selected media without actually invoking the media picker (i.e. I can't retrieve previously-selected PHPickerResult items, only the IDs)
• [x] Stretch: Allow videos

Composer

• [x] Fix @mention typeahead
  • This will involve creating a decodable struct to represent (or translate) v1 user objects
• [ ] Stretch: Add custom string attributes for hashtags, cashtags, mentions, and URLs to display in the composer

Lil bits

• [x] I'm using built in materials now but they look broken on the status bar and action bar; maybe try ultra thin material?
• [x] The tweet character count has regressed; it seems to be counting 140 chars
• [ ] Add share sheet support