Bug 11210: Writeoff partial amounts

Bug 11210: Writeoff partial amounts

Minor changes to pay.pl and paycollect.tt to allow writing off a partial amount of a fine.

Test plan:
0) Go to the Fines tab of a test patron's profile
1) Create a fine if there are none (under the Manual invoice tab)
2) Go to the "Pay fines" tab
3) Press the write off button on the corresponding account line

Without patch, you'll be asked to confirm, but will not be able to edit the amount
With patch, you'll be able to edit the amount.

Followed test plan, patch worked as described. Also ran QA test tool and
modified files passed

Signed-off-by: Simon Pouchol <simon.pouchol@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>