Enable "Mobile Notifications" when a site is registered

Requirements for testing:

• Ngrok

Testing:

• Install a new site with this branch
• Give the site a public internet site address (URL) using Ngrok
• As admin, access to the site using the public URL
• Go to Site administration -> Messaging -> Notification settings and check that you see the "Mobile" plugin enabled (checkbox with a tick) just under Notifications plugins
• Click on the Settings link in the "Mobile" row
• Go to the bottom of the page and click in the "Request access key" link and confirm that:
  • You see an error "Your site is not yet registered"
• Now, click on the "Register your site" link just at the right of the error
• And complete the registration process by accepting the "Privacy notice" and clicking "Register your site" button
• Once the registration process finishes, go back to the Site administration -> Messaging -> Notification settings > Mobile settings
• Go to the bottom of the page and confirm that:
  • The "Airnotifier Access key" field is not empty
• Copy that key somewhere else (in a notepad or somewhere else)
• In the same page, remove the "Airnotifier Access key" field content and click Save changes
• Now click in the link "Request access key" and confirm that:
  • You successfully retrieve the access key and it is exactly the same you annotated before
• Once you have finished testing, you can unregister the testing site via Site administration -> Registration and them clicking "Unregister your site" link at the bottom