Make PHP API tests clean up after themselves

Most tests were cleaning up after themselves properly, but a few
weren't. They are now.
WIP on #35: Placeholder for answer editor

The actual editor, and hooking it up to the API, will come later. But
now when we click on the edit link we see the placeholder -- and only
one placeholder will be in the DOM at any given time.
Fix ng-jqte directive that was not binding in both directions.
Add answer editing.

Everything appears to work now. There's a minor bug where if user A
creates an answer and then user B edits it later, the answer is seen
as "owned" (created) by user B after the edit goes through. But that's
a separate issue, to fix later. The important thing for now is that
editing works.

Fixes #35.