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.
* TODO for the meta_topic_id
* TODO for the root plugin site setting key, so all settings don't end
up in the Plugins category
* TODO for the translation of the plugin category name, which maps
to the root plugin site setting key
