Break AphrontDatabaseConnection dependencies on PhabricatorEnv

Summary: We pull "retries" and a doc link from PhabricatorEnv directly. Break these dependencies so the classes can move to libphutil.

Test Plan: Browsed site, triggered a schema exception and verified I still got the useful footer text.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1283

Differential Revision: https://secure.phabricator.com/D3053