Add createdBy and created parameters when creating a group

When a group is created the `createdBy` and `created` parameters are currently missing. They need to be stored in Cassandra and returned in the group profile, just like content already does.

Assigning to @stuartf for implementation.
I think `created` can be an `int` or `timestamp` here.

We used `text` for our other date values mostly out of ignorance and oversight. AFAIK there isn't a good way to migrate a column's datatype in Cassandra (yet) so we're stuck with them unfortunately. It would be good if we could use the proper datatype when introducing new columns.