Change rest-api namespace to wp-notifications/v1

[johnhooks]

What problem does this address?

The plugin should not use the core wp/v2 namespace

What is your proposed solution?

@TimothyBJacobs suggested wp-notifications/v1 in #179, which sounds good.

Another thought is to stick with the "Notify" name and do wp-notify/v1, this is shorter, and if this does merge into core, the "wp-notify" namespace/prefix used everywhere would simply become "wp".

[Sephsekla]

This is good call, and I think having a version in there is smart.

I'd prefer to go with wp-notifications over wp-notify, we switched over for a reason and we should try and be as consistent as we can.

[johnhooks]

@Sephsekla with this change, /wp-json/wp-notifications/v1/notifications seems like an odd endpoint. How do you feel about /wp-json/wp-notifications/v1/messages? In consideration that it would be part of:

/wp-json/wp-notifications/v1/messages
/wp-json/wp-notifications/v1/channels
/wp-json/wp-notifications/v1/subscriptions

Also the class name with namespace would be more descriptive. Example:

WP/Notifications/Message and WP/Notifications/Message_Controller

Rather than:

WP/Notifications/Notification and WP/Notifications/Notification_Controller

Just thoughts. I'm leaving it as /wp-json/wp-notifications/v1/notifications unless I hear otherwise.