Add support for the ability to validate *stagings* over RPC rather
than via webhook. This may later be expanded to PRs as well.
The core motivation for this is to avoid bouncing through github which
sometimes drops the ball on statuses, and it's frustrating to have a
staging time out because GH fucked up.
Implemented via RPC, requiring both the staging itself (by id) and the
head commit being affected, as that is necessary to know what CIs are
required for that head and correctly report cross branch on the
various PRs.
Fix#881 (kinda)
