Add ability to change an instance default theme from the administration panel (#7092) (#8381)

* Add default_settings class method to ScopedSettings

ScopedSettings was extended to use value of unscoped setting instead of
only using defaults set in config/settings.yml for selected settings.
This adds possibility for admins to set default values of users' settings,
for example default theme (as requested in #7092).

* Add ability to change an instance default theme

Closes #7092

app/controllers/admin/settings_controller.rb

@@ -16,6 +16,7 @@ module Admin
       timeline_preview
       show_staff_badge
       bootstrap_timeline_accounts
+      theme
       thumbnail
       hero
       min_invite_role