R1:dd501117e80c

When deleting inline comments, offer "undo" instead of prompting

Summary:
Ref T2009. Ref T1460.

Fixes T2618. When users hit "Delete" on inline comments, delete immediately and offer them "Undo". If they delete indirectly (e.g., by clicking "Delete" from the preview at the bottom of the page), we still prompt them, because the "Undo" action either won't be available or may not be easy to find. This is a "refdelete".

Fixes T6464. This was just a mess. Make it not as much of a mess. It should work now.