LP#1745427: account for change in prediction patterns

LP#1745427: account for change in prediction patterns

This patch fixes a bug in the web staff client where changing
the active prediction pattern for a subscription would not
reset the base issue when predicting the next run of issues.

To test
-------
1. Set up a subscription in the web staff client with a prediction
   pattern (e.g., monthly).  Predict some issues.
2. Create/select a new prediction pattern for a new publishing schedule,
   e.g., quarterly.
3. Activate the new pattern and deactivate the old pattern.
4. Click Predict New Issues.
5. Enter in the new publication date/enumerations/chronology as needed.
6. Enter a prediction count and select Save. Note that new issues
   appear to be based on the previous pattern.
7. Apply the patch, delete the issues created in step 6, then repeat
   steps 4-6.  This time, new issues should be created using the new
   prediction pattern.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>