Support lessc include paths and global variables in extension packaging.
This improves extension packaging by allowing .less files to include

files relative to one of the registered static directories, and to make

use of global variables.
When building a .less file, Djblets will calculate all the main static

media directories provided by the parent project and by the extension

and add them to the less import search path. This allows an extension

to, do, say:
@import "myapp/css/defs.less";

It also makes use of the new global variable support in LessCSS 1.5.1

to pre-define variables for .less files. BuildStaticFiles subclasses can

override get_lessc_global_vars to define the global variables available

to a .less file.
This can be used to standardize search paths in a way that works during

both runtime (using less.js) and compile time (with lessc), since those

two modes otherwise have very different search paths. For example:
@import "@{DEFS_PATH}/mixins.css";

It can also be used to define variables used for LessCSS guards, to

separate rules for runtime vs. packaging.

Tested an extension in Review Board that imported our defs.less file

using a global variable, and defining that variable to a URL in a template

for less.js, and a location in a search path for packaging (in the

custom BuildStaticFiles command).
I was able to successfully make use of definitions in both cases.
Verified in the generated .css files that the definitions were used,

and no imports remained.