Implement VAPID support

[marco-c]

Fixes #120.

[marco-c]

Looks like it's working. I'm using default values for the audience (origin of the WordPress site), subject (email address of the administrator), private key (generated when installing the plugin), so VAPID will be enabled by default.

I'd like some help with the wording (names and description of the fields):

@digitarald @groovecoder @jrconlin can you help?

[groovecoder]

Wow, that's great to see! Here's the text blurbs I'm using to describe VAPID to dev dashboard users:

To monitor your push application messages, your application server must implement VAPID. VAPID allows your application server to submit information about itself to the push service, which improves application stability, exception handling, and security.

See the VAPID Draft for more details.

On this push app side of the experience (i.e., generating the VAPID public key), I've added this copy to the WebPushDataTestPage:

To monitor your push messages to Firefox, you need to add your app and its VAPID public key to the Mozilla push dashboard. To validate you own the app, you will also need to sign a token with your VAPID private key, and paste the signed token back into the dashboard.

If you are using this page to test, you can paste this VAPID public key into the dashboard:
Then paste the app token here to get the signed token value:

This last bit can/should probably be a step-by-step instruction list? I.e. ...

To monitor your push messages to Firefox

1. Add your app and its VAPID public key to the Mozilla push dashboard. You will receive a token value to sign
2. Paste the token value here to sign it with your VAPID private key: <input type="text">
3. Paste this signed token value back into the dashboard.

[groovecoder]

An amazing experience would be a simple button: "Click here to register this push application with the Mozilla Push Dashboard"

[groovecoder]

Could we always show this top of the VAPID table, regardless of whether or not it's supported by the PHP platform? I.e., include just the description and link to VAPID docs with a message like "VAPID requires PHP 5.5+, mcrypt, gmp, and openssl extensions."

[marco-c]

Done in a1bee7c.

[marco-c]

There's a list now, satisfied prerequisites are green, unsatisfied red.

[marco-c]

I'll take care of the integration with a dashboard in a follow-up.