Hide `new merge request` button from index page

When a project is archived, don't show the button on the index page
of merge requests

app/views/projects/merge_requests/index.html.haml

 - @no_container = true
 - @can_bulk_update = can?(current_user, :admin_merge_request, @project)
-- merge_project = can?(current_user, :create_merge_request_from, @project) ? @project : (current_user && current_user.fork_of(@project))
+- merge_project = merge_request_source_project_for_project(@project)
 - new_merge_request_path = project_new_merge_request_path(merge_project) if merge_project
 
 - page_title "Merge Requests"

spec/features/projects/merge_requests/user_views_open_merge_requests_spec.rb

@@ -94,6 +94,18 @@ describe 'User views open merge requests' do
       end
 
       include_examples 'shows merge requests'
+
+      it 'shows the new merge request button' do
+        expect(page).to have_link('New merge request')
+      end
+
+      context 'when the project is archived' do
+        let(:project) { create(:project, :public, :repository, archived: true) }
+
+        it 'hides the new merge request button' do
+          expect(page).not_to have_link('New merge request')
+        end
+      end
     end
   end