users: rework privacy mode into private favorites (fix #4257).

* 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.

app/views/users/_post_summary.html.erb

@@ -11,7 +11,7 @@
   </div>
 <% end %>
 
-<% if presenter.has_favorites? %>
+<% if presenter.has_favorites? && !user.hide_favorites? %>
   <div class="box user-favorites">
     <h2>
       <%= link_to "Favorites", posts_path(tags: "ordfav:#{user.name}") %>