Bug 20819: Add your consents tab to opac-user

Bug 20819: Add your consents tab to opac-user

This patch adds a consents tab to the OPAC user account menu. We now
add a GDPR section here, but it is open for future extensions. Think of
a newsletter checkbox for instance.

Script opac-patron-consent handles the tab. And now only includes some
GDPR code but is also written for more general use too.

Test plan:
[1] Set GDPR_Policy pref to Disabled. Verify that OPAC operates as usual.
[2] Set pref to Permissive. Try to save a consent or a refusal. Note that
    you are not logged out when saving a refusal.
[3] Set pref to Enforced. Save a refusal. You should be logged out.
    Log in again and verify that the consents tab shows a No.
    Note: a follow-up patch will add further enforcements.

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>