Create a basic push notification console

[jchambers]

This still needs a lot of work, but the big pieces are there and, if you're very very careful, it will actually send push notifications!

TODO:

• [x] Add form validation to make sure we have all the fields we need before trying to send a notification (note to self: -fx-text-box-border seems to be the thing we want)
• [x] Use a fixed list of topics when using certificates
• [x] Move strings to a resource bundle for localization
• [x] Save recent values in a Preferences store
• [x] Protect against button-mashing on the "send notification" button