Bug 27410: (follow-up) Updates to accommodate currency entry

Bug 27410: (follow-up) Updates to accommodate currency entry

This patch makes two modifications in order to allow the entry of
currency strings (e.g. 1.25).

 - Inputmode changed to "decimal"
 - Regex pattern changed to "^\d+(\.\d{2})?$" as found in
   admin/debit_types.tt

To test, apply the patch and test entry of various strings in the manual
invoice "amount" field. Confirm that the field only accepts digits or
digits followed by a decimal and two digits.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>