Bug 26283: Add dateexpiry and dateenrolled to new borrower fields modal

Bug 26283: Add dateexpiry and dateenrolled to new borrower fields modal

While these may not make sense for all patron field preferences, I think
it's better to have them in general than to have them missing.

To test:
- Check the modal for BorrowerMandatoryField system preference
- Verify dateenrolled and dateexpiry fields are missing
- Apply patch
- Verify the dates are listed now
- Set both to mandatory
- Verify that they are indeed mandatory when adding a new patron now

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>