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/models/form/admin_settings.rb

@@ -30,6 +30,8 @@ class Form::AdminSettings
     :show_staff_badge=,
     :bootstrap_timeline_accounts,
     :bootstrap_timeline_accounts=,
+    :theme,
+    :theme=,
     :min_invite_role,
     :min_invite_role=,
     :activity_api_enabled,