RFC: Allow duplicate emails

[pA1nD]

RFC

Problem

I've seen many people having difficulties with their email accounts (not being able to access them or only have a shared one).

This issue tackles the scenario in which a family only has one email account but want to create multiple accounts for the family members.

Proposal

Allow multiple users being created using the same email address.
This can be achieved via simple Keycloak configuration as described here: https://issues.redhat.com/browse/KEYCLOAK-4059.