This adds the initial infrastructure using Redux for centralizing the UI
state as an immutable object accessible via `annotationUI.getState()`
which is updated as a result of actions from the UI, network etc.

For background on why we want to do this, see the design overview at
https://github.com/hypothesis/h/pull/3176

Additionally this commit removes a couple of tests that checked for
non-mutation of the selected/focused annotation maps and uses
seamless-immutable instead which provides a better guarantee of this,
but only in debug builds.