Refactor group privacy settings (rename viewable_by to privacy)
Merge branch 'feature/group-privacy-settings' into feature/subgroup-viewable-by-parent

Conflicts:
	app/assets/stylesheets/groups.css.scss
	app/helpers/groups_helper.rb
	app/models/ability.rb
	app/models/permitted_params.rb
	app/views/groups/_form.html.haml
	app/views/groups/group_setup/setup.html.haml
	config/locales/en.yml
	db/schema.rb
	features/groups/privacy/parent_viewable_group.feature
	features/groups/privacy/public_group.feature
	features/groups/privacy/secret_group.feature
	features/step_definitions/group_privacy_steps.rb
	spec/controllers/groups_controller_spec.rb
	spec/models/ability_spec.rb