R1:509fbb6c20e2

When building audit queries, prefilter possible "authorPHID" values

Summary:
Ref T13244. See PHI1057. Currently, if you're a member of a lot of projects/packages, you can end up with a very large `commit.authorPHID IN (...)` clause in part of the "Active Audits" query, since your `alice` token in "Responsible Users: alice" expands into every package and project you can audit on behalf of.

It's impossible for a commit to be authored by anything but a user, and evidence in PHI1057 suggests this giant `IN (.