R1:17caecdda35d

Make workboard real-time updates mostly work

Summary:
Depends on D20654. Ref T4900. When a task is edited, emit a "workboards" event for all boards it appears on (in a future change, this should also include all boards it //previously// appeared on, and all parents of both sets of boards -- but I'm just getting things working for now).

When we receive a "workboards" event, check if the visible board should be updated.

Aphlict has a complicated intra-window leader/follower election system which could let…