Bug 34585: Add data-code attribute to chargeperiod_charge_at selector

Bug 34585: Add data-code attribute to chargeperiod_charge_at selector

When one tries to edit circulation rule "When to charge"
columns value is always set as "End of interval". This patch adds
data-code attribute to chargeperiod_charge_at selector
to define existing rules value.

To reproduce:
1. Navigate to Circulation and fine rules page.
2. Find or create a rule with a "When to charge" value set as
"Start of interval".
3. Edit rule.
=> Note that "When to charge" value is now "End of interval",
while as the other values should be copied as they were.
4. Apply this patch.
5. Edit rule again.
=> "When to charge" should now be copied correctly.

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>