The new pipeline changes broke any extension-provided .less files that
needed to include or reference anything from another module, along with
any global variables provided by the app. There were also problems
loading static media from installed packages.

The arguments to lessc were still being set in the old setting, which
Pipeline no longer respects, and it wasn't in a suitable format (it
needed to be a list of strings, not a string).

This fixes that setting, plus ensures that we don't have broken
conflicts between what the app must now provide for normal use and
what's needed for packaging. It does this by filtering out app-provided
settings, replacing them with ours.

It also disables the use of the special Djblets LessCompiler's import
script for checking outdated dependencies. This isn't needed during
packaging, and in fact fails due to static file storage access control
checks.

To fix the package static media, we now have specialized versions of the
Pipeline CSS/JavaScript template nodes that change the rendering
behavior to fall back on outputted (compiled) static media files if the
sources are unavailable. This depends on a pending change in Pipeline
(https://github.com/jazzband/django-pipeline/pull/552).

Tested against Power Pack, which wasn't working with the new Pipeline

changes due to referencing stylesheets in Review Board. I was able to

successfully compile them without problems.
Tested loading Power Pack pages when installed via a package, and

tested when the package was in develop mode. Wiped my htdocs/static/

tree each time, and verified that the page was loading the appropriate

files. Also verified that if I modified a file (source or output,

depending on which was being used), the changes were picked up when

next reloading the page.
Existing unit tests pass.