When users submit "editing" inlines, warn them that their inlines will be saved

Summary: Ref T13513. This slightly expands the existing-but-hacky "warning" workflow to cover both "mentions on draft" and "submitting inlines being edited".

Test Plan:

• Submitted changes to a revision with mentions on a draft, inlines being edited, both, and neither.
• Got sensible warnings in the cases where warnings were appropriate.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21191