Bug 28012: (QA follow-up) Use form validation for numbering patterns

Bug 28012: (QA follow-up) Use form validation for numbering patterns

Activates standard validation and error messages. Converts from
pattern check to digits class for "display order" field.

To test:
* Compare error message when leaving required fields empty
* Compare error message when entering wrong/false data in display order

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit f816027ea15278c7ed4b738915e963e22ee5c2e6)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>