Minor improvements on IssuableActions

CHANGELOG

app/controllers/concerns/issuable_action.rb

app/controllers/concerns/issuable_actions.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/models/ability.rb

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

app/views/shared/issuable/_form.html.haml

db/schema.rb

doc/api/issues.md

doc/api/merge_requests.md

lib/api/issues.rb

lib/api/merge_requests.rb

spec/controllers/projects/issues_controller_spec.rb

spec/controllers/projects/merge_requests_controller_spec.rb

spec/requests/api/issues_spec.rb

spec/requests/api/merge_requests_spec.rb