add json "config"-like field to EditLog model

[whatisgalen]

It would be nice to add a kind of miscellaneous JSON config field to accommodate project developers who wish to capture other information on the edit log instance. I don't think the Arches model should be exhaustive but it should permit developers to reasonably capture relevant data without having to override the model itself (as project migrations can be a headache)

[chiatt]

Do you have an example of what data somebody might want to collect?