Raise a lint warning for classes not marked "abstract", "final" or "@concrete-extensible"

Summary:
See T795. I think ~all classes in the classtree should be "abstract" or "final",
but I provided "@concrete-extensible" if you really have "Rectangle extends
Square extends Shape" or something.

I'm not totally sure this should be enabled globally by default, maybe I should
default it to DISABLED and then enable it for libphutil/arcanist/Phabricator? It
feels like it might be a little overbearing to push on everyone by defualt.

Test Plan: See unit tests.

Reviewers: btrahan, aran, nh, arudolph, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T795

Differential Revision: https://secure.phabricator.com/D1510