Create an issue for all unresolved discussions in an MR

What does this MR do?

This MR builds a new issue for unresolved discussions in an MR. When creating the issue, all the discussions are resolved in the originating MR. The discussions get a new system note mentioning the newly created issue.

The following still needs to be done:

• Add a checkbox to the Issue form to resolve all discussions in the originating MR
• Resolve all discussions in the MR
• Add a system note for each resolved discussion

Are there points in the code the reviewer needs to double check?

Am I iterating over the correct discussions or should I only mention the diff_discussions

What information should the built issue contain?

Why was this MR needed?

This would allow the reviewer of an MR to quickly resolve all discussions in an MR to be able to merge it after !7125 (merged) is implemented.

Screenshots (if relevant)

This currently just includes a simple link when applicable with the merge request status:

But this is still being discussed in #23589 (closed)

When clicking this link a new issue is prepared with references to the unresolved discussions:

At the bottom of this page we have a mention that this will resolve all discussions in the merge request.

But this should be a bigger warning as mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/23589#note_17481832

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

The discussion was started in #23589 (closed) This is based on !7125 (merged) (which was started in !6385 (closed)), so I'll rebase those first commits out when it gets merged.

@DouweM @smcgivern