[FIX] (website_)mass_mailing: remove website dependency (once and for all?)

mass_mailing snippets contained links to "/page/contactus", url which
only exists if website is installed. Also, t-if="website" which were
in snippets were unclean.

Now, the website_mass_mailing module which was already there to install
website snippet and which was already in auto-install with website and
mass_mailing is also in charge of extending mass_mailing features. It
extends the snippets to add the parts which were marked with t-if
website and add the links to "/page/contactus".