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.

test/unit/post_test.rb

@@ -1706,7 +1706,7 @@ class PostTest < ActiveSupport::TestCase
         @parent = FactoryBot.create(:post)
         @child = FactoryBot.create(:post, parent: @parent)
 
-        @user1 = FactoryBot.create(:user, enable_privacy_mode: true)
+        @user1 = FactoryBot.create(:user, enable_private_favorites: true)
         @gold1 = FactoryBot.create(:gold_user)
         @supervoter1 = FactoryBot.create(:user, is_super_voter: true)