Repeat SSO user lookup by emails in username/nickname fields

When logging in with e.g. OpenID Connect/Azure, two email addresses
may be provided, one in "email" field and another in "username" or
"nickname". Since this is exactly the opposite of what the separate
Azure backend does, migration from Azure to OpenID Connect/Azure
needs to try both emails which may be different.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>