Framework: add schema for current user

[gwwar]

This adds a schema for current-user to avoid data shape changing errors, as described in #3101 Also tidies up a test to use sinon rather than mockery.

Testing Instructions

• Start Calypso with:
  ENABLE_FEATURES=persist-redux make run
• In the console set localStorage.debug to calypso:state
• Refresh the page a few times and navigate to a few sections.
• The app behaves normally and loads the persisted state/current-user from IndexedDB storage

cc @rralian @aduth @artpi @mtias

[aduth]

Arguing the smallest of points, but I wonder why ESLint doesn't identify this as needing a semi-colon? In my mind it should I'd think.

[gwwar]

;) Looks complicated: eslint/eslint#2194 (comment)

[aduth]

Looks and works well for me 👍

On an unrelated note, do you have a recommended reference for JSON Schema aside from the website? The specs are... well, specs 😄

I do this tool handy for validating from a schema: http://jsonschemalint.com/

[gwwar]

👍 @aduth thanks for reviewing!

On an unrelated note, do you have a recommended reference for JSON Schema aside from the website?

I've been a bit of a magpie and have been looking at a lot of misc docs. If I find a particularly good one I'll be sure to share it.

It does look like there's some nice generators that take in sample JSON and return a generated schema. This might be useful if we have a particularly large object.