Comment on file header in merge request diff

Resources

PM @victorwu | UX @cperessini | FE @psimyn

Problem

See https://gitlab.com/gitlab-org/gitlab-ce/issues/2641, https://gitlab.com/gitlab-org/gitlab-ce/issues/24816, https://gitlab.com/gitlab-org/gitlab-ce/issues/20390 as relevant discussions.

Description

• View and add comments to a file in a merge request diff.
• This applies for both text files and image files.
• For example, in here (gitlab-com/www-gitlab-com!4522 (diffs)), we would like to allow users to add comments to the new images.
• In here (dimitrieh/comment-on-pictures!2 (diffs)), we would like to allow users to add comments to the changed images.
• The comments should be associated with an image diff or a text file diff.
• The comments are resolvable, just like line diff comments.

Design

The comment thread is below the file header, but above the file content.

Availability and Testing

Add feature spec to ensure that file header comments can be added immediately, added to a review, published and deleted.