Make "hidden" changesets sticky, and show hidden state in the filetree

Summary:
Ref T13455. Make "hidden" a changeset property similar to other changeset properties.

We don't need to render this on the server, so we make a request (to update the setting) and just discard the response.

Test Plan:

Maniphest Tasks: T13455

Differential Revision: https://secure.phabricator.com/D21158