* 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

* @object is not needed

* Remove unneeded dependencies

* Do not call private method

* Prefer #respond_to_missing? over #respond_to?

`#respond_to?` doesn't support `User.settings.method(:method_name)`

* Use find_or_initialize_by instead of