Updated the notify module to prevent it from relying on model.py.

The general strategy will be to pass in lists of email/link data,
instead of passing in keys and leaving it up to the notify methods
to use the model.
Now addPollParticipants prevents the addition of duplicate participants.

Also, getParticipantVoteLinks accepts a participants key-email map
instead of just the poll_key. This makes it easier to call the same way
for both the initial poll creation, and when adding additional
participants.