Transactional 'batch' merge request comments

Moved to https://gitlab.com/gitlab-org/gitlab-ee/issues/1984

As a reviewer every time I press the 'Add comment' button in a merge request every subscriber gets and email which might annoy them.

I want prevent notification overload by grouping comments, stage comments and submit them (email people) in the end.

Possible solutions:

1. Have 'Draft comment' and 'Submit comment' buttons
2. Have checkbox or review mode 'bundle comments'
3. Something else

Phabricator solved this already.

Thanks to Eric Vander Weele