Our main Pipeline CSS bundles (admin, common, and reviews) now

include a single file for the Review Board codebase, which is

responsible for importing all other files in the bundle.
This has a number of advantages:


Rules can be more easily combined and deduplicated, such as when

  extending rules.


Relative paths (for image files) can be simplified, since all files

  now come from a central place.


We'll be able to use Less plugins that can optimize rules across the

  resulting CSS file.


There are fewer files to load per page during development.


This has required some cleanup in places. @import statements now use

the absolute (e.g., rb/css/...) paths in all cases, which appears to

avoid some issues with how LessCSS determines whether it's processed a

file.
A lot of old path-relative code (for url()s) has also been replaced,

and paths made standardized with some new variables in

rb/assets/paths.less.
All bundle entrypoints live in css/bundles/, rather than having a

directory per bundle. There are two reasons for this:


We'll probably end up merging bundles together soon. Maybe most of

   them. It'd be ideal not to have to keep moving files around.


Extensions can import these files as references, to make use of

   functions or variables, making these hard to move.


This ends up shrinking the bundles quite a bit:

admin drops from 45KB to 44KB
common drops from 359KB to 246KB
reviews drops from 146KB to 129KB

Tested the dashboard, review request page, diff viewer, and various

pages in the admin UI.
Tested building the package.