Activities (WIP)

I may want to enforce that each Event only belongs to max 1 Activity of each type.

My opinion is that if 2 activities have the same platform, person, resource, attempt AND type, they should be combined into 1, because they refer to exactly the same thing. And Events also specify all of those fields, so by that logic they only belong to up to 1 Activity of each type. But we should discuss this.