When a user clicks "Cancel" on an inline comment to leave the "Editing" state, save the state change

Summary:
Ref T13513. Now that the "currently being edited" state of inlines is saved on the server side, clear the flag when the user clicks "Cancel" to leave the "editing" state on the client.

This also serves to delete empty comments.

Test Plan:
  - Clicked a line number to create a new comment. Then:
    - Clicked "Cancel". Reloaded page, saw no more comment.
    - Typed text, saved. Reloaded page, saw non-editing draft. Clicked "Edit", reloaded page, saw editing draft. Clicked "Cancel", reloaded page, saw non-editing draft.
    - Typed text, saved. Clicked "Edit", deleted text, saved. Reloaded page, saw no more comment.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21187