R1:74b438db136f

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…