refactor: convert to namespaces

[johnhooks]

What?

Implement naming convention to discussed in Issue #197.

Why?

There is currently a few differently systems of naming across the project and it should be consistant.

How?

• Remove WP_Notify class prefix
• Remove wp-notify file prefix
• Convert to namespace WP\Notifications

Also cleans up some misaligned PHPDoc comments.

[johnhooks]

This is a first step to fix naming in the PHP code. The only code removed are the serialization tests. They are problematic. The previous system of serializing class instances into JSON that is initialized based on class names embedded into the JSON won't fit well with standard WordPress rest-api schemas.

[Sephsekla]

In general there's a lot that needs doing still in terms of documentation etc - there are a lot of classes etc which need commenting (and as discussed, maybe some things we can remove).

Added one thought around namespace imports, let me know what you think.

[johnhooks]

@Sephsekla I agree, this is just a first step. How would you like to work through the whole process?

I would prefer doing it in stages so each PR is simple in purpose and easier to review. This one is rather large, but I think its narrow scope makes it easy to review and understand.

[johnhooks]

I checked everything and this is ready for review.

[erikyo]

I have searched around all files but there is no more trace of wp-notify! Good job @johnhooks and thanks, now we have all the files consolidated 🚀