CSS plan stage one complete: rearchitecture and custom stylesheet support

[Myk Melez]

Developers and reviewers,

With Gerv's reviews and Dave's approvals (thanks Gerv and Dave!), I 
checked in three patches today to 2.20 that together finish implementing 
the first stage of the Bugzilla CSS plan.

The patches move Bugzilla CSS files from css/ to skins/standard/, make 
checksetup.pl create skins/custom/, and include skins/custom/ 
stylesheets in Bugzilla pages.  These mainly architectural fixes 
implement support for custom (installation-specific) stylesheets and 
pave the way for future support for third-party skins.

If you are a Bugzilla developer who does CSS hacking, look in 
skins/standard/ for the files to hack.

If you are a Bugzilla reviewer reviewing a patch from before the change, 
the patch should apply cleanly to the equivalent files in 
skins/standard/.  Just replace the css/ directory in your Bugzilla home 
directory with a symlink to skins/standard/ before applying the patch:

cd <bugzilla home directory>; mv css css.old; ln -s skins/standard css

-myk