This optimises how GroupProjectsFinder builds it collection, producing
simpler and faster queries in the process. It also cleans up the code a
bit to make it easier to understand.
* Meld the following disparate endpoints:
* `/projects/:id/events`
* `/events`
* `/users/:id/events`
+ Add result filtering to the above endpoints:
* action
* target_type
* before and after dates
