audit-logs: Add index to RealmAuditLog for realm and event type.

[laurynmm]

Adds an index on RealmAuditLog for the realm, event_type, and id in order to improve database queries on these audit logs.

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[timabbott]

Probably worth putting this one first in the list, since it's the primary index.

[alexmv]

The id is in there for ordering purposes, and as discussed elsewhere, event_time is probably better here -- and we should audit for existing order_by clauses to make sure they're doing them by that, and not id.

[timabbott]

I made the changes to use event_time and tested that this index was indeed also used even when one ordered by ID, since filtering to (realm, event_type) is usually pretty precise. With those changes, merged, thanks @laurynmm!

We should probably still do the audits that @alexmv suggested, and also look at adding parallel indexes to some remote audit logs, but this will do for this PR.