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/