This updates the json_dumps template filter to always sort keys,

providing a consistent result that won't differ based on Python version

or other factors.
This change may regress some unit tests that were checking serialized

content against a specific version of Python's sorting order, but for

broader Python compatibility, those have to be fixed anyway.

Djblets unit tests pass. Review Board tests are impacted, but will

be addressed in a future change (landed alongside this one).