View and add comments to file 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 (https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/4522/diffs), we would like to allow users to add comments to the new images.
• In here (https://gitlab.com/dimitrieh/comment-on-pictures/merge_requests/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.