Create a new issue for a single discussion

What does this MR do?

Based on what is shown in #25515 (moved) I want to make it possible to create a new Issue for a single discussion in a merge request.

It also adds a button to resolve all discussions mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/23589#note_18864573

• Add a button to resolve a single discussion to the merge request view
• Pre-fill a new issue when passing a a discussion to resolve
• Resolve the discussion when creating the issue
• Add a system note to the discussion mentioning the new issue
• Hide the button after the issue was resolved
• Add button to resolve all discussions in an MR

Why was this MR needed?

This is the backend part that is required to complete #25515 (moved).

Screenshots (if relevant)

The warning on the new-issue form looks like this for all discussions in an MR:

And like this for a single discussion:

There's a new button to create a new issue for a single discussion:

And there is a button to delegate all discussions in a merge request

After the issue is created, this is mentioned in a flash message:

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?

#25515 (moved)