Bug 31935: Fix misaligned form when adding serial subscription

Bug 31935: Fix misaligned form when adding serial subscription

This patch attempts to fix the misaligned form on subscription-add.tt by removing some of the wrapping <p> tags and adding labels where appropriate.

To test:
1. Add a serial subscription and notice the misaligned form on step 1.
2. Apply patch
3. Look again and confirm that the form looks better.
4. Read through everything and make sure the form still makes sense.
5. Make sure the form still works properly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>