[IMP] mail: allow tracking on groups protected fields

Purpose
=======

Currently fields with a `groups` attribute can't be tracked.
Otherwise changes would be visible by all in the chatter, including
users which normally don't have the access rights because they are
not members of `groups`.

Specification
=============

Filter tracking field values according to the `groups` attributes.
Users without the access rights should not see them.

If a message is only composed of tracking values and the user doesn't
have the rights to see them, an empty message should not be displayed.