Serials planning: Update subscription edit to properly deal with date changes. (...

Serials planning: Update subscription edit to properly deal with date changes. ( patch 3 / 3 )

Previously subscription-add.pl allowed modification of 'firstacquidate', which changed
the subscription definition, but did not affect prediction.  This patch adds two fuctions
to Serials.pm to get/set the current expected issue date (note that all date calculations
in prediction patterns are based on the current expected date, and there's only one serial
issue per subscription in the 'expected' status at any time).  Subscription editing
now allows you to edit the next expected date, but not the first acqui date (unless you
haven't received any issues yet), thus allowing for adjustments in the prediction pattern.
This patch also updates fixes some discrepancies in irregularities / prediction display.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>