The Features change broke all unit tests that deal with
`SiteConfiguration`. This happened because `djblets.features`, which
is put into `INSTALLED_APPS`, would import `djblets.features.checkers`,
which imported `SiteConfiguration`.

It's not valid for a module in `INSTALLED_APPS` to directly or indirectly
import a model. While this didn't actually break things in Django 1.6,
it does in 1.7 and higher, due to more strict behavior for model
loading. This import ended up preventing `SiteConfiguration` from being
properly synchronized to the database.

To fix this, the SiteConfiguration import has been moved into the
function that needs it.

There were also two other HTML-related failures.

1) There was a check in the conditions tests that broke due to the
   addition of newlines in generated HTML for one of the widgets we use.
   We now use `assertHTMLEqual()` to perform a more suitable check.

2) We allowed use of an older Pygments releases, but tested against
   newer HTML. We're now requiring a more modern version in
   dev-requirements.txt.

Testing Done:
Unit tests pass against Django 1.6 and 1.8.

Reviewed at https://reviews.reviewboard.org/r/8300/

File dev-requirements.txt need changes on line 9