Work around SLF4J thread-safety issues.

Before, we were ignoring log messages if two loggers were initialized
around the same time. Now we aren't.

Unfortunately, if some dependency (e.g., BoneCP) requests a logger at
the same time as we do (in a separate thread), we'll still have the
problem.
Don't log stuff from org.overviewproject in tests
Refactor Vizs to pave the way for _real_ Vizs

Right now, Tree is the only viable Viz. But it isn't a _real_ Viz. So I
created a trait, VizLike, and used it in the controller and view code.

I also included a bunch of half-finished Slick code. It's missing an
evolution. It doesn't harm anything.