Rewrite relative date formatter function

Aeosynth-approved, or at least acknoledged
Reimplement relative post times in namespace

Less quick-and-dirty, more right.

Still need to polyfill visibility
Polyfill and correct usage of page visibility

Reselecting the tab always flushes (since ui thread block will be
percieved as part of the tab switch), but background flushes don't queue
when tab is not visible.

This is actually a better implementation than html5chan has right now,
will need to backport it later.