Refactor the post preview html to use the ViewComponent framework. This
lets us encapsulate all the HTML, CSS, and helper methods for a UI
component in a single place.
See https://viewcomponent.org.
* Rename 'privacy mode' to 'private favorites'.
* Make the private favorites setting only hide favorites, not favgroups
and not the user's uploads on their profile page.
* Make the favgroup is_public flag default to true instead of false and
fix existing favgroups to be public if the user didn't have privacy mode
enabled before.
* List _all_ public favgroups on the /favorite_groups index, not just
favgroups belonging to the current user.
* Add a /users/<id>/favorite_groups endpoint.
Make /favorites redirect to a ordfav:<user> search instead of having a
separate view just for favorites. This duplicated a lot of code for no
good reason.
