R1:d46378df208f

Modernize "Responsible Users" tokenizer and add "exact(user)" token

Summary:
Ref T10939. Fixes T9263. Ref T4144.

First, this resolves users (converting users into all packages and projects they are responsible for) earlier, so bucketing can act on that data correctly. Previously, your own blocking reviews would appear in "Must Review" but your packages/projects' would not. Now, all of them will.

Second, this adds `exact(username)` to mean "just me, not my packages/projects". You can use this along with…