Remove the Pygments runtime dependency checks.

We have some really old code (Makefile days) for checking if syntax
highlighting can be enabled, based on having Pygments, and having a
minimum version of it. This code, in combination with some other code,
ends up creating some ugly and unnecessary circular dependencies between
parts of the product that only appear if you start importing certain
modules first.

These days, we have actual dependencies that get installed with the
product. This check is no longer useful, and causes more harm than good.
This change removes the check and all calls.

Reviewed at https://reviews.reviewboard.org/r/5811/
Condense some unnecessarily verbose code in reviewboard.admin.

Due to changes made over time to the admin widget code, we ended up
having some code in reviewboard.admin.signals that didn't really belong
there. A signals.py file should be fairly minimal and contain actual
signals, but this one had just signal handlers that were called from
reviewboard.admin itself.

This change just removes the file and moves that code directly into
reviewboard.admin, to keep things simple and consistent.

Reviewed at https://reviews.reviewboard.org/r/5812/