Server Side Rendering: Move global app state to a (flux) ApplicationStore

[hoverduck]

Outtake from discussion at 9287-gh-calypso-pre-oss

Accept section prop as well here. Normally the section is set via context.layout.section I think, which is an anti-pattern.

I think you mean context.layout.setState(), but I agree that it is something that should probably change. We probably should have an AppStore or something that holds global app state that can be updated via actions. As it is, we're basically treating the Layout instance as singleton.

I like that. It would be cool to tidy the getDefaultContext() data into an AppStore.

[ockham]

We now have a setSection Redux action and corresponding reducer, so I'm closing this. (No immediate need for anything else from getDefaultContext() to convert to a Redux subtree.)