Audit log database design changes

In order to improve performance and support more filtering options (&3179 (closed)), this issue is about working on a first design proposal.

We can start with the existing features and come up with design changes to support those better. We should consider how the additional filtering would work, but we can leave implementing that for a later time.

Statistics

# select entity_type, count(*) from audit_events group by 1;
 entity_type |   count   
-------------+-----------
 Group       |   8493694
 Project     |  74313841
 User        | 217208835
(3 rows)