Enforce valid email address

[jgonggrijp]

• Add email addresses to all existing accounts.
• Add email addresses to Accounts->Email addresses, link to users.
• Require email address confirmation for new registrations.

[BeritJanssen]

This is very work-intensive if done for both acceptance and production server. Requiring email addresses entails the following settings:
ACCOUNT_EMAIL_VERIFICATION = "mandatory" ACCOUNT_EMAIL_REQUIRED = "True"
After changing this setting, everyone whose email address is not in the admin interface cannot log in anymore. This will be especially a problem for student testers who may have been added through the admin interface, rather than signed up themselves.