Bug 20819: Db rev for new table patron_consent and sysprefs

Bug 20819: Db rev for new table patron_consent and sysprefs

[1] We add table patron_consent.
It allows for future extension. We could e.g. save consent for newsletter etc.
[2] Add one field to borrower_modifications
Datetime the patron who self-registered, gave consent.
[3] Syspref PrivacyPolicyURL
[4] Syspref GDPR_Policy: enforced, permissive or disabled.

Test plan:
[1] Run the dbrev or do a new install.
[2] Look at the description of the new prefs in Administration.

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>