Improve application settings and write tests

app/controllers/admin/application_settings_controller.rb

app/controllers/application_controller.rb

app/controllers/registrations_controller.rb

app/controllers/sessions_controller.rb

app/helpers/application_helper.rb

app/helpers/application_settings_helper.rb

app/models/user.rb

app/services/base_service.rb

app/services/gravatar_service.rb

app/views/admin/application_settings/_form.html.haml

app/views/admin/application_settings/edit.html.haml

app/views/admin/application_settings/show.html.haml

app/views/layouts/devise.html.haml

app/views/layouts/nav/_admin.html.haml

config/routes.rb

features/admin/settings.feature

features/steps/admin/settings.rb

features/steps/shared/paths.rb

lib/gitlab/current_settings.rb

spec/models/application_setting_spec.rb