Speed up audit processing on user profile

Prefetch instance to remove additional queries object_cache lookups by
instance

Use object caches for audit display_name

This cuts out 35 queries for an admin user on my dev box with about 15
instances. Brings the in-database time down from ~350ms to ~160 ms.
Speed up get_user_instances

Fewer queries, simpler operations

Shaves off about 20-30 ms on an admin user in my dev box with 15
users. Makes the query timing waterfall diagram more balanced, for what that's worth.
Defer evaluation of querysets

This results in an additional speedup despite the additional (fast)
query, probably because these querysets are being modified again anyway before being
re-evaluated for rendering.