This was confusing people last year, most people assumed it meant
the tags they used the most / categories they posted in the most.
This should clear things up.
Also removes the `isRewindActive` function in favour of
`is_rewind_active`
on the current user, since then we only have one place to check this,
and
it makes testing easier since we don't have to mock browser time.
Finally it moves the route for rewind in the UI to the new route path
format.