Fix for Bug 6534 - Duplicate opac renew buttons at the top of the checkouts list

Fix for Bug 6534 - Duplicate opac renew buttons at the top of the checkouts list

This patch uses JS to add two links to the top of the checkouts
table: renew selected and renew all. Clicking each one submits
the respective form.

Also added is a small validation routine to warn the user if
they try to "renew selected" without having checked any boxes.

Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>