Add `feature` tag to feature specs

Not to be confused with the RSpec `type: :feature` tag, this tag is used
by the `spec:feature` Rake task for filtering/grouping specs.

spec/features/groups_spec.rb

 require 'spec_helper'
-feature 'Group' do
+feature 'Group', feature: true do
   describe 'description' do
     let(:group) { create(:group) }
     let(:path)  { group_path(group) }

spec/features/issues/filter_by_milestone_spec.rb

 require 'spec_helper'
-feature 'Issue filtering by Milestone' do
+feature 'Issue filtering by Milestone', feature: true do
   include Select2Helper
-  let(:project) { create(:empty_project) }
+  let(:project) { create(:project) }
   before do
     login_as(:admin)

spec/features/login_spec.rb

 require 'spec_helper'
-feature 'Login' do
+feature 'Login', feature: true do
   describe 'with two-factor authentication' do
     context 'with valid username/password' do
       let(:user) { create(:user, :two_factor) }

spec/features/markdown_spec.rb

@@ -32,7 +32,7 @@ require 'erb'
 #
 # See the MarkdownFeature class for setup details.
-describe 'GitLab Markdown' do
+describe 'GitLab Markdown', feature: true do
   include ActionView::Helpers::TagHelper
   include ActionView::Helpers::UrlHelper
   include Capybara::Node::Matchers

spec/features/merge_requests/filter_by_milestone_spec.rb

 require 'spec_helper'
-feature 'Merge Request filtering by Milestone' do
+feature 'Merge Request filtering by Milestone', feature: true do
   include Select2Helper
   let(:project) { create(:project) }

spec/features/notes_on_merge_requests_spec.rb

 require 'spec_helper'
-describe 'Comments' do
+describe 'Comments', feature: true do
   include RepoHelpers
   describe 'On a merge request', js: true, feature: true do

spec/features/password_reset_spec.rb

 require 'spec_helper'
-feature 'Password reset' do
+feature 'Password reset', feature: true do
   def forgot_password
     click_on 'Forgot your password?'
     fill_in 'Email', with: user.email

spec/features/profiles/preferences_spec.rb

 require 'spec_helper'
-describe 'Profile > Preferences' do
+describe 'Profile > Preferences', feature: true do
   let(:user) { create(:user) }
   before do

spec/features/projects_spec.rb

 require 'spec_helper'
-feature 'Project' do
+feature 'Project', feature: true do
   describe 'description' do
     let(:project) { create(:project) }
     let(:path)    { namespace_project_path(project.namespace, project) }

spec/features/users_spec.rb

 require 'spec_helper'
-feature 'Users' do
+feature 'Users', feature: true do
   scenario 'GET /users/sign_in creates a new user account' do
     visit new_user_session_path
     fill_in 'user_name', with: 'Name Surname'