registration-extra-fields: Selectively require/hide registration fields

Extend the capabilities of the REGISTRATION_OPTIONAL_FIELDS
configuration variable, to allow to select, for each individual field,
if it should be 'hidden', 'optional' or 'required'.

Rename the configuration variable to REGISTRATION_EXTRA_FIELDS to reflect
the additional capabilities, and updates the defaults.

Ticket: MCKIN-168
registration-extra-fields: Add country/city to registration & UserProfile

As extra fields, configurable through the REGISTRATION_EXTRA_FIELDS
variable. Hidden by default.

Ticket: MCKIN-184