Phame - allow blogs to specify custom URIs

Summary: this then enables people to create blog.theircompany.com. And for us, blog.phacility.com...!

Test Plan:

• created custom URIs of various goodness and verified the error messages were sensical.
• verified if "false" in configuration then custom uri stuff disappears

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3542