Move "Mailing Lists" to a separate application

Summary:
There's currently no way to get here from the UI since nav tabs don't exist anymore. It's also always been hard to find this feature even when we had the tabs, since it's surprising that it's inside "MetaMTA".

• Move mailing lists to a separate application.
• Add buildApplicationPage(), since we don't really need buildStandardPageResponse() any more -- we can infer all the information from PhabricatorApplication. This will let us get rid of a lot of the PhabricatorXXXController classes which just define application information.
• Add getApplicationURI() to reduce code duplication, and in case we want to let you move applications around some day.

Test Plan: Looked/edited/saved mailing lists.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T631

Differential Revision: https://secure.phabricator.com/D3248