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.

Referenced our defs file in a new extension's .less file. Tested it with

the extension both in development mode and as a packaged extension, and

manually verified the compiled .css file.
Played around with variables depicting the globals set, and guards using

the version and DEBUG information, and verified they're working right.