move task_list setup and events to own class

What does this MR do?

deduplicate some task list code. Will need some more changes to work with Notes' task lists

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

Clicking checkbox on a Note used to submit the form. This meant the note was marked as edited. Now it uses PATCH (same as issue/MR) so ticking boxes only updates description, and doesn't flag as edited

Also doesn't need a changelog, that was created mainly for science. Can delete

Why was this MR needed?

less code

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?

#27246 (closed)