Fix the triangle entity for IE9.

IE9 doesn't understand &blacktriangledown;, so we have to use the
numeric code instead. This at least works cross-platform.

Fixes bug #3017
Fix compatibility with Python 2.5.

The latest RB release broke Python 2.5 compatibility, due to using
@property.setter, which is only in 2.6+. This fixes it to be compatible
with 2.5.

It also sets our markdown dependency to hard-code a working version for
Python 2.5.
Fixed another broken drop-down triangle in IE9.

This now covers both the Support and account links.
Release Review Board 1.7.11.
Merge branch 'release-1.7.x'
Add periods after API error strings.

Some of the error strings had periods, and some didn't. I haven't yet checked
djblets' errors, but this makes the reviewboard errors consistent.
Add DIFF_PARSE_ERROR to the ValidateDiffResource.create error list.

This corrects the errors listed in the docs.