Bug 15774: Add permission for managing additional fields

Bug 15774: Add permission for managing additional fields

To test:
 1 - Have three borrowers, one with order_manage permission, one with
 edit_subscription permisson, andone with both
 2 - Apply patch, updatedatabase
 3 - Verify all three now have the manage_additional_fields permission
 4 - Visit the admin page with these users, they should all see the
 'Manage additional fields' link
 5 - Click the link
 6 - User with order_manage should see 'Order baskest'
 7 - User with edit_subscription should see 'Subscriptions'
 8 - User with both should see both
 9 - Remove the additional permissions from a user - they should see a
 note about needing additional permissions

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>