Provide pre-defined variables for .less files.

We now supply development and packaged .less files with a set of global
variables they can use when packagin. These provide a base path for
static files (to allow for importing defs and other files), debug
information, and version information, which can be used for LessCSS
guards to supply different rules for different versions of Review Board.

There's also a fix for building .less files. We didn't have
FORCE_BUILD_MEDIA set, causing issues building .less files, depending on
the development setup.