Add Notifications: when primary subjects are all retired, when predictions change significantly

[lcjohnso]

Investigate methods for notifying research team (here, Galaxy Zoo team) and Zoo team in the case that:

1. Most primary (not spice / random) subjects placed in target subject set are retired and no longer getting served to volunteers.
   1. The motivation: when most target subjects are retired, it is time for the research team to evaluate whether to continue classifying the current subject set or not.
   2. The completeness fraction for the target workflow on the subject set resource could be checked programmatically -- when greater than 0.95 (or similar), notify project team or flag to Zoo team.
2. Predictions from the model change significantly from week to week.
   2. The motivation: some results from the weekly runs are available to the research team, but it would be helpful to have "push" notifications if model predictions change significantly -- meaning that the project team should take a look and potentially change how training is run.