Allow uuid primary keys for activities table

[shijithkjayan]

This lets others use spur in their projects even when they use :uuid for their primary key.

[tfwright]

Hi, thanks for opening this. I'd like to get some tests in place for this before merging, which will be a bit of a pain because it will require a new test schema. If you'd like to give it a shot, feel free, otherwise I should have some time coming up to add those.

[shijithkjayan]

I can give it a try