Split mapStateToProps into smaller functions, one for each container

[fzzzy]

In the Storybook documentation there is some rationale for why to do this:

Each container will have its own mapStateToProps and mapDispatchToProps that provide the bare minimum state and actions required for the view to function, eliminate any potentially-expensive changes to the DOM from irrelevant changes to the store.

We should spend some time determining which containers would benefit from having their own mapStateToProps.

[fzzzy]

Also, by definition a container in react has a unique mapStateToProps etc., so if we find that almost all state for the entire app is passed to every container we currently have, we may want to look into adding new containers that require a narrower scope of state.

[lmorchard]

Something else that occurred to me while poking at flow types: Could help to organize the output of these functions into types, even if they don't group cleanly into per-container types.

For example, I started splitting out some into groups that relate to shared topics common to subcomponents used by multiple containers somewhere down in the rendering tree. That would at least help document what all these things are for. Like, every per-component mapStateToProps would probably need a SendToGAProps type, but we can dry that up.

[fzzzy]

@lmorchard That seems really useful! Should help a lot with clarifying what state goes where.

[chuckharmston]

This might be useful for #3405, but otherwise I'm going to leave this as stretch.