Change PHUIPagedFormView to extend AphrontView

Summary: Debugging crumbs in repository editing, and it seems there are stray divs that aren't used from extending AphrontTagView. I don't see any specific reason this needs to be from AphrontTagView, so changing it. Of course I'm not sure this is correct, so feel free to reject if I'm missing some obvious or non-obvious reasons.

Test Plan: Review editing a repositor, don't see extra div.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11608