JSONField was prone to unwanted queries when working with models

that defferred the JSONField. This wasn't so much a major problem on

Django 1.6, but manifested on newer versions of Django. The core issue

was that, during the model initialization phase, JSONField would try

to load data from the model in order to normalize it, and if the field

was deferred, this act would trigger a query. To fix this, this change

now checks whether the attribute is deferred before performing any

lookups.
Other changes to the parent TextField in newer versions of Django

broke other aspects of JSONField. TextField.get_prep_value() was

added, which forced the value to be a string, and in our case this

resulted in some unwanted serialization. We now override this, instead

of get_db_prep_value() (which will, in the default implementation,

calls our new method and do the right thing).
On the other end of that process is the to_python() method, which we

weren't overriding. We now provide this, and have it return a

deserialized representation of the data.

Djblets and Review Board unit tests pass.