Add concept of "skins" to phame, and add a phacility skin

Summary:
introduce an abstract "PhameBlogSkin" class and instantiate two versions -- PhabricatorBlogSkin (Default) and PhacilityBlogSkin.

Most notable hack is including the directory /rsrc/images/phacility - this lets things "work" without messing around with the phacility.com CSS and instead just cutting and pasting most of the file.

Test Plan: played around with Phame a bunch. In particular, created a blog with a custom domain and the phacility skin. Verified it looked good and individual posts looked okay.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3687