Use different format for meetings in firebase

[charwking]

This PR accomplishes a couple things:

• Prevents non-admin users from accessing meetings data in firebase
• Removes the unused formatDate method in meetingApi.service.spec.js
• Changes the meetings model to be a "true" firebase array -- with a generated guid as the key -- and stores the date as a property on the meeting. It also makes sure the dates are stored in yyyy-mm-dd format so that they can be sorted if necessary.

In order to (manually) convert existing meetings to this new format, the following function can be used:

function change(ref, key, date) {
   ref.child(key).once('value', function (data) {
      var obj = data.val();
      obj.date = date;
      ref.push(obj);
   });
}