Releeph: use relative URIs

Summary: I used the `PhabricatorEnv::getProductionURI()` in too many places to build Releeph URIs.  The only places that should need full URIs are the links generated for Releeph emails, and in Conduit responses that link to Releeph objects.

Test Plan:
- Grep for `getProductionURI()` in Releeph, and make sure only sensible, non-DOM building places use it.
- Inspect the Releeph DOM to make sure hrefs etc. are relative.

Reviewers: wez, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5999