Properly represent JSON data in text fields in the admin UI.
Our JSON support has historically had some issues in the admin UI,

though we broke it a bit further in a recent release. This change

finally makes it work the way we always dreamed it would.
JSON data is now properly shown as-is, not as a Python repr() of the

structure, complete with validation. What's saved becomes valid JSON in

the model.
To make this work, JSONField now uses a custom form field

(JSONFormField) to do the conversion at the appropriate time from the

Python data to the serialized JSON. This will automatically be used in

every part of the admin UI and in every form that uses a JSONField.
Note that as part of this, I've backed out the previous change intended

to fix this. It actually regressed the functionality by concealing any

exceptions raised during JSON validation and turning the whole thing into

an empty dictionary, which was pretty bad. It also used the new-style

except Exception as e syntax, breaking Python 2.4 and 2.5.

Unit tests pass.
I checked several database entries and saw valid JSON.
Tried entering invalid JSON and got a useful error message telling me

what was wrong.
Entered valid JSON and it saved correctly.