WIP: Bulk editing no subscribe#24589

What does this MR do?

When a user performs a bulk-update of issues, they become subscribed to every affected issue. This removes that side effect by overriding their subscription status.

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

You will see in the code, that a bunch of issues are collected for which their subscription status will be undone. That is likely to be an N+1 query. I'd love some assistance to help prevent that.

I'm also not sure this is the best implementation. I'm wondering if perhaps removing the initial side-effect would be a better solution rather than overriding it, after-the-fact.

Why was this MR needed?

To solve: https://gitlab.com/gitlab-org/gitlab-ce/issues/24589

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [-] NA -- Documentation created/updated
• [-] NA -- 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?

#24589 (closed)