Modernize MetaMTA

Summary:

• Add an Application.
• Move routes to the application.
• Move nav out of tabs (which no longer exist).
• Fix a couple of random things.

Test Plan: Viewed sent/received mail logs. Performed send/receive tests. Viewed email details.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T631, T1569

Differential Revision: https://secure.phabricator.com/D3255