Mark line comments on a MR as resolved.

This would be a button that is shown on any line comments of a MR.

[Resolve] / [Resolved]

It would be visible on non-collapsed comments, but also on collapsed comments.

Discussion out of date but unresolved / Discussion out of date and resolved

We can have a counter on top, showing how many of the total comments are resolved, with a button to resolve all line comments

[Resolve all line comments] 4/10 line comments resolved

Should automatically email the subscribers when everything is resolved

Job resolved all line comments on MR 123

cc @sytses @dzaporozhets