Fix unicode problems in BaseReviewRequestDetails.__str__

The BaseReviewRequestDetails __str__ method was returning unicode, even for
python 2. Most of the time this wasn't an issue, since we don't use that method
very much, but if the summary contained unicode characters, it could break the
admin UI. Making this class use the @python_2_unicode_compatible decorator
fixes this.

Testing done: Ran new unit test.

Fixes bug 3325.

Reviewed at https://reviews.reviewboard.org/r/5770/
Fix dependency checks in manage.py

Our dependency checks (which print errors and warnings when running the
devserver) weren't working, because of the changes in how the settings module
is defined. We used to check whether the settings module was defined in the
environment, but when we switched to Django 1.5, we started inserting that into
the environment ourselves. I've added a flag to check whether or not we're in a
subprocess.

Testing done:
Ran the devserver and saw it run the dependency checks (but only once).

Reviewed at https://reviews.reviewboard.org/r/5770/
Merge branch 'release-2.0.x'