Fix #205, fix #156, fix #124

Fix #233, fix #268

skip ActionCable for follow/unfollow/block events, instead clear UI from
blocked account's posts instantly if block request succeeds. Add forgotten
i18n for sensitive content

resources that require a user context vs those that don't (such as public timeline)

/api/v1/statuses/public   -> /api/v1/timelines/public
/api/v1/statuses/home     -> /api/v1/timelines/home
/api/v1/statuses/mentions -> /api/v1/timelines/mentions
/api/v1/statuses/tag/:tag -> /api/v1/timelines/tag/:tag

This makes the UI appear less laggy. After all, the status *is* created
right there and then, it's the distribution to home timelines that can take
a while

all statuses/accounts to assemble, but at least lists don't have to be
re-rendered all the time now. Also add "mention" dropdown option

Also fix reloading of timelines after merge-type events

entire timelines (only fetch since last known ID). Side effect: account
timelines no longer update in real-time

registration API