There might be many Email object with same email, but as they are subcontent
of other types, the unicity key shall be extended with parent type.

They were lost during migration from filesystem based constraint.

Changing field name in the form means that the field itself should also be
renamed.

For example, on Mandriva, if SECURE_TMP is enabled (in /etc/security/shell),
then TMP/TMPDIR are set to $HOME/tmp but $HOME is only accessible by the owner
and its group, but not slap user, then the extends-cache temporary directory
will not be accessible after dropping the privileges.