Framework: add schemas for themes state tree

[gwwar]

This adds JSON Schemas to themes, so we can avoid data shape change errors as described in #3101

• I persisted every sub-tree in themes except themes-list because of transient isFetchingNextPagestate. We can persist this subtree after that is moved someplace else.
• It doesn't appear that theme-details is currently in use.

Testing Instructions

• Start Calypso with:
  ENABLE_FEATURES=persist-redux make run
• In the console set localStorage.debug to calypso:state
• Navigate to calypso.localhost:3000/design
• Refresh the page.
• Themes behaves normally and loads the persisted themes data from IndexedDB storage

Bonus:

• Invalidate one of the themes schema by adding required: [ 'foo' ]

{
    type: 'object',
    required: [ 'foo' ],
    properties: { 
         //.... 
    }
}

• Navigate to calypso.localhost:3000/design
• Refresh the page.
• A console warning is printed, letting us know that themes had some invalid data.
• The Themes page still loads and works normally.

cc @ockham @aduth @artpi @retrofox @mtias @rralian

[gwwar]

@seear @aduth @rralian This is ready for another look

[seear]

It doesn't appear that theme-details is currently in use.

theme-details is now in use (in dev and wpcalpyso.wordpress.com) though is very much 'in progress' with the schema likely to change, so we should probably not persist this until the feature is finished.

[gwwar]

Thanks for the update @seear, I'll rebase this and not persist theme-details after going through review pings.

[gwwar]

Closing this out due to time elapsed. I'll start fresh with a different PR