Use custom LESS compiler.

Review Request #7988 — Created Feb. 22, 2016 and submitted — Latest diff uploaded


Review Board


This switches us over from pipeline's LessCompiler to Djblets'. The major
advances here are that we now get on-demand recompilation of only the files
which have changed (or have had their import dependencies changed), and we can
have source maps. This means that reloads are quite a bit faster, and that the
styles listed in browser devtools reference the correct lines of the .less file
rather than the compiled .css output.

I've also fixed up the include path to properly reference
reviewboard/htdocs/static, which has the latest collected/compiled sources
from all packages, including any activated extensions.

Verified that things were compiled and loaded correctly. Changed defs.less and
saw all the dependent files get recompiled. Reloaded without any CSS changes
and saw the browser get the page very fast.