insights: explore pre-calculating permissions for regular users

[coury-clark]

We currently perform repo permission filtering in the query path by finding the users permissive repos and filtering from the set of matching repos for a time series. We should explore what it might look like to pre-aggregate insights for regular users (power users). Some questions:

1. What heuristics can we use to detect who to preaggregate for?
2. What kind of limits can we place (count, size, etc)?
3. Is it worth it?

/cc @Joelkw @felixfbecker @vovakulikov