Trash can for deleted comments

Some organisations want to be able to keep comments indefinitely, even when deleted.

As such, we should allow keeping a per-page trashcan for comments. When a comment is deleted, it's placed in this trash, and a "view deleted comments" link appears in the page operations bar. Space admins can view/purge/restore these comments per page.

Trashed comments do not show up in the space-level trashcan (too much clutter), except as a count at the top (also 1234 deleted comments), and an option for additional purging (Purge all... (Including comments)).

The existence of a comment trash should be optional (and default to off), as it imposes an unnecessary manual management burden on the general use-case, which is that comments are ephemeral and their deletion not really much of an issue.