Bug 16795 - Patron categories: Accept integers only for enrolment period and age...

Bug 16795 - Patron categories: Accept integers only for enrolment period and age limits

To test:
- Apply patch
- Clear browser cache
- Go to Home > Administration > Patron categories
- Create or edit a category
- Verify that field 'In months', 'Age required' and 'Upperage limit'
  accept digits only

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit e3002eea202a50b68e96650f37bcacc37843872e)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>