Add demo REST controller

[Sephsekla]

What?

This PR adds a demo REST Controller for fetching notifications.

Why?

This allows us to move the demo content to the PHP side, allowing real js querying of the endpoint.

How?

Testing Instructions

Screenshots or screencast

[Sephsekla]

Hmm, looks like an issue with PHPCS now, don't think it's related to my code but I'll take a look later.

[erikyo]

I think we need to add
excludes: 'tests/'

into actions WPCS action template. (check here)

[Sephsekla]

I think we need to add excludes: 'tests/'

into actions WPCS action template. (check here)

Hmm, weird that this test passed on the previous commit though.

[erikyo]

indeed i was wrong, it is not an issue of with excluded paths and looking closely even files inside "./includes/" have the same problem.

if you have the time try updating wpcs, the one we are using is an old version