fix(controller): don't encode empty JSON objects or arrays as strings

The django-json-field 0.5.5 package has a bug encoding empty JSON objects.
Instead use a subclass which fixes this as a special case.
Merge pull request #1616 from mboersma/1563-fix-empty-json

fix(controller): don't encode empty JSON objects or arrays as strings
fix(controller): store limit fields on config

Having a separate "Limit" domain object was getting in our way, when
really it represents just an extension of a Config. This moves the
two JSON fields "cpu" and "memory" to Config and does away with Limit.