Update Activity Record Metadata

Ref: https://github.com/maiera/gde-app/issues/32

Tested against my local dev environment and the endpoint methods insert/list/get work nicely with the metadata.

Can someone check if I got all the properties covered or if I have missed something?

Also, do we want to add some sanity checks in the ActivityRecord.insert method or do we handle this on the front-end?
We probably should call this type or activity_type to go along with our nomenclature everywhere else.