Implement Merge Request Labels

app/assets/stylesheets/sections/merge_requests.scss

app/controllers/application_controller.rb

app/controllers/projects/labels_controller.rb

app/models/merge_request.rb

app/models/project.rb

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

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

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

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

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

...iews/projects/merge_requests/show/_participants.html.haml

app/views/shared/_project_filter.html.haml

lib/api/entities.rb

lib/api/merge_requests.rb

spec/requests/api/projects_spec.rb