This introduces reviewboard.admin.server.get_data_dir(), which returns

the the absolute path to the data directory.
In practice, this is always $HOME, whether in production or

development. This is important, as much of the state stored and accessed

in this directory is SCM-specific, which hard-code $HOME. However,

that also means that for development, state gets written to the user's

$HOME, but that's not a new problem.
Our checks have been updated to use this, and some upcoming code will be

using this as well.

Unit tests pass.
Made sure that startup checks still work.
Made use of this in some in-progress code.