Bug 33197: Rename GDPR_Policy system preference

Bug 33197: Rename GDPR_Policy system preference

GDPR is a European Union (and, at time of writing, UK) law.
The GDPR_Policy system preference is about a patron
giving consent to their personal data being processed in
line with the library's privacy policy.

The name of the preference is vague: there could be
many policies implemented by libraries to comply with
GDPR. It also makes the preference look irrelevant for
libraries outside the areas where GDPR applies, while
it may be useful for libraries anywhere.

This renames GDPR_Policy to PrivacyPolicyConsent and
adjusts the system preference descriptions.

To test:
* Apply the patch
* Run database update
* Search for GDPR_Policy in the system preference
  - you should not find anything.
* Search for DataPrivacyConsent in the system preferences
  - you should find it and be able to activate it
* Verify the feature works as expected
  - If the preference is set to "enforced", you will be
    asked to give consent to the data privacy agreement
    in the OPAC when you log in
* Verify the page is now phrased neutrally using 'privacy policy'

Bonus: Consent date is now formatted according to DateFormat
       system preference.
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>