Bug 1026685 - Silent and priority notifications for CMAS alerts r=mhenre...

...tty

* add a list of applications whose notifications should be silent
* add a list of applications whose notifications have more priority and stay on
  top of the notification screen
* Also reorganized some unit tests
Seems a little dirty, would rather use the `extensions.gaia.domain` pref, or expose window.GAIA_DOMAIN in the build step. But, in reality the code you have above is the most robust.