Bug 20882: Move items.uri to mediumtext

Bug 20882: Move items.uri to mediumtext

Applies to items and deleteditems.

Test plan:
Run new install or upgrade. Check field size.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
(cherry picked from commit 853d0ee468d1f5ba1efb3a9746855486f4d25e6c)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>