redis-backed "mentions" timeline as redundant (given notifications)

nested inside a common parent (<Account>), instead they all embed <HeaderContainer />

Closes: #345

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

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

This prevents the bug where if you go "back" to the UI after navigating to
another page it loads with the old set of statuses

Also, only update relative time every minute instead of 6 seconds. My bad

timeline reload in UI, other small fixes

Mostly resolves #8, though attachments are still not displayed in public view