[VERY WIP] Redux refactor

[markspolakovs]

Move away from SET_XYZ Redux actions and to more XYZ_HAPPENED style - because firing multiple actions for the same "action" is a bit of a code smell (and it makes the action history harder to parse, and potentially less consistent - and also you have various bits of redux firing actions which conceptually belong in other modules).

At this point it's just a trial run, with BroadcastState, which is relatively small.

[mstratford]

This definitely looks interesting, but is defo a big change in style. I have feelings about whether I like the more verbose "do this thing", rather than having random other parts of the codebase triggering by themselves based on action firings.