mod_mailinglist: New dialog which lets you combine lists

Using this new dialog, the recipients of two lists can be combined
according to the three set operations union, subtract and intersect.

Also add documentation for the mailinglist recipients admin page.
(cherry picked from commit 1afbaa28d16780ba1da089c14ea2a4435dab1248)

Conflicts:
	modules/mod_mailinglist/translations/es.po
	modules/mod_mailinglist/translations/nl.po
	modules/mod_mailinglist/translations/pl.po
	modules/mod_mailinglist/translations/tr.po