Bug 20819: Add consent to self-registration process

Bug 20819: Add consent to self-registration process

We add a section for the GDPR consent in opac-memberentry (only for the
self-registration). Not when editing personal details.

Test plan:
[1] Enable selfregistration (with confirm) and GDPR policy.
[2] Register a new account in OPAC. Verify that the GDPR checkbox is
    required.
[3] After you submit, you should see a date in borrower_modifications
    field gdpr_proc_consent.
[4] When you confirm, verify that the consent is visible on your consents.
[5] Enable selfregistration without confirmation mail. Register again.
[6] Check your consents tab again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>