Feature: reopen closed merge request

app/assets/stylesheets/sections/merge_requests.scss

app/controllers/projects/merge_requests_controller.rb

app/models/concerns/issuable.rb

app/models/issue.rb

app/models/merge_request.rb

app/observers/merge_request_observer.rb

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

app/views/projects/merge_requests/show/_mr_box.html.haml

app/views/projects/merge_requests/show/_mr_title.html.haml