Bug 14978: Add unique key constraint on issues.itemnumber

Bug 14978: Add unique key constraint on issues.itemnumber

The constraint might have existed before bug 14069.
For these DB, this patch will restore it, for others, it will add it :)

Test plan:
> show create table issues;
should not return "unique key itemnumber"
Execute the updatedb entry twice
> show create table issues;
should return only one "unique key itemnumber"

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>