allow deactivating old grants without removing them

[graik]

Currently, all the grants added to the grants table are always listed in the order form. However, over time, grants end and cannot be used any longer. Deleting them is not an option as this would also delete the associated orders.

ToDo:

• Add a "disabled" field to the grant datamodel
• Hide grants that are disabled from the radio button list in the order form

[graik]

perhaps more logical to call the field "active" which is consistent with e.g. the auth.user model.