Make users theme configurable

GITLAB-1262

Change-Id: I690cb8ea294df53ebe8405a519c23c501af2c21a

Conflicts:
	app/models/user.rb
	config/initializers/1_settings.rb
	spec/models/user_spec.rb
Defaulted theme to MARS

Change-Id: I02814fb47edcc915309183547cc8dcb165100f4f