Add config setting to disable mandatory tokens check in mailings

As per a previous discussion (with Lobo IIRC), add a configuration
setting to disable the check for presence of mandatory tokens when
sending mailings.

This setting is implemented as a negative option, i.e. ticking the
option disables the check. We could also reverse the logic, so that
ticking the option enables the check instead; and set it to on by
default on installation and upgrade. However, this would complicate the
patch considerably -- and I'm not even sure it would really be
preferable to have it work that way around...