Pass through the 'sort_by' when clicking delete/hide.

That way, after the delete or hide is done, the page comes back the
way it was before, rather than re-defaulting to sorting by creation
time.

Auditors: chris

Test Plan:
uploaded to appengine and clicked on 'sort_by_last_snippet_time and
clicked on the button to hide someone, and the page came back sorted
by last snippet time still.

Also, ./snippets_test.py