Add option to create merge request when editing/creating a file

app/assets/javascripts/blob/edit_blob.js.coffee

app/assets/javascripts/blob/new_blob.js.coffee

app/assets/stylesheets/pages/editor.scss

app/controllers/application_controller.rb

app/controllers/projects/blob_controller.rb

app/helpers/merge_requests_helper.rb

app/views/projects/blob/edit.html.haml

app/views/projects/blob/new.html.haml