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.

db/migrate/20200118015014_change_is_public_default_on_favorite_groups.rb

@@ -0,0 +1,6 @@
+class ChangeIsPublicDefaultOnFavoriteGroups < ActiveRecord::Migration[6.0]
+  def change
+    change_column_default :favorite_groups, :is_public, from: false, to: true
+    add_index :favorite_groups, :is_public
+  end
+end