Use literal_eval in JSONField when necessary.

JSONField would sometimes end up dealing with a string that was encoded
as a repr() of the object, instead of as a JSON serialization. In this
case, we used to use eval(). However, that's not ideal, and isn't very
secure.

Python 2.6+ offers literal_eval, which does what we need and is safer.
Older versions do not. For those versions, we ship the literal_eval
function from Python 2.7.

We also do a better job of sanitizing the resulting content before
returning to the caller in this case, helping to prevent failures with
loading pages and fixing the problem.
Release Djblets 0.6.30.