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

• Fix media attachments
• Allow alt text
• Allow multiple photos
• Allow gifs
• 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)
• Stretch: Allow videos

Composer

• 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

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