Make new DiffInline code handle most "delete" operations

Summary:
Ref T12616. This moves the delete actions to the new, more stateful way of doing things.

These are a little tricky because you can click "Delete" on an inline, but you can also click "Delete" from the preview area at the bottom of the page. If you do, the inline you are deleting may or may not be present on the page.

This has a few bugs -- notably, deleting from the preview without interacting with the on-page inline first won't actually delete the on-page inline yet -- but nothing too serious.

Test Plan: Deleted inlines, undid deletion, deleted from preview.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17890