Bug 28390: Move timestamp into grouping row

Bug 28390: Move timestamp into grouping row

This patch moves the timestamp that was getting repeated for each
transaction breakdown row into the group header row.

Test plan
1/ Add a series of transactions to to a register (via Point of Sale
or/and Borrower Accounts)
2/ View the transactions (and past transactions tables) on the Register
details page
3/ Apply the patch and compare the display

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>