[#6384] Add admin button to delete annotations

[HelenWDTK]

Relevant issue(s)

Fixes #6384

What does this do?

Adds comment deletion to the request admin and comments admin pages, for single annotations and bulk deletion

Why was this needed?

Sometimes annotations need deleting entirely to comply with GDPR. This stops admins needing to nag the devs to do it.

Implementation notes

Screenshots

Pro Admin timeline view: Normal admin view: Deletion event log: Comments admin page Request admin page: Single annotation edit page

Notes to reviewer

I think this is almost ready now, but there's undoubtedly room for improvement.