Support editing target branch of merge request

CHANGELOG

app/models/merge_request.rb

app/services/issuable_base_service.rb

app/services/merge_requests/update_service.rb

app/services/system_note_service.rb

app/views/projects/_issuable_form.html.haml

features/project/merge_requests.feature

features/steps/dashboard/dashboard.rb

features/steps/project/merge_requests.rb

spec/services/merge_requests/update_service_spec.rb

spec/services/system_note_service_spec.rb