State: Extract selectors to a global selectors file

[youknowriad]

closes #693

To factorise some selectors and test them easily. I've moved all the redux selectors into a separate file and add unit tests.

[aduth]

Selectors are a thing we consciously avoided by worry about boilerplate and indirection. I like that you've taken a mild approach with a single selectors.js file. Testing selectors seems very valuable. I'd be okay with this, but curious to hear if others have concerns.

[aduth]

We can assume isPostSaveRequestFailed will return a boolean and drop the coercion

[nylen]

Selectors are a thing we consciously avoided by worry about boilerplate and indirection.

To address this, how about we make a rule (or rule of thumb) that we only extract something into a selector if it's more complicated than just a simple pull from state? For example, this doesn't seem to add much value:

export function isEditedPostDirty( state ) {
	return state.editor.dirty;
}

[aduth]

There is some value in treating state as an opaque shape with selectors as its isolated entry points; specifically that it leaves open the option to restructure the state object without requiring sweeping changes.

[nylen]

I don't have a clear preference either way. Also, even if some of the tests are trivial now, this won't always be the case, and the additional testability will serve us well for any future refactoring of the state tree.

I'll rebase and merge this 👍