Require Django Evolution 0.7.1 or higher.

This release fixes issues that some users encountered when upgrading
from old Review Board databases on MySQL.
Merge branch 'release-2.0.x'
Split up parts of defs.less and common.less.

This is the first step in cleaning up our CSS rules, with the aim of
bringing some consistency, organization, and to make some rules more
reusable for extensions.

defs.less's mixins have been moved out into a set of files in mixins/.
defs.less pulls those in, meaning that any existing consumer of
defs.less will continue to work.

common.less has too much junk in it. Some of the things in there are
reusable UI components, like boxes, buttons, and banners. Those three
have been pulled out into files in a ui/ directory.

My goal is to get all the styles into consistent places over the next
few changes, and then begin to standardize some of the naming, hopefully
coming up with a standard set of classes that we can document and reuse
in extensions.

Reviewed at https://reviews.reviewboard.org/r/5859/