Aprhont - Adding cookie-prefix, as config option, and into cookie methods

Summary: Cookie-prefix should fix phabricator instances where x.com and x.y.com have conflicting cookie names

Test Plan: Pushed branch to dev.phab.example.com, logged into phab.example.com and into dev.phab.example.com.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7979