Add a curated posts page at /explore/posts/curated. Curated posts are
the most favorited posts by contributor-level users (users with
unlimited upload permissions).
Also add an order:curated tag using for use in regular searches.
* Fix the next button for the weekly timescale to jump to the next week,
not the next day.
* Show the start and end dates for the weekly timescale.
* Use `Time.zone.today` instead of `Date.today` to respect the user's
timezone setting.
