templates/filters: extracting the user portion of an email address
Currently, the 'user' filter is using util.shortuser(text) (which clearly
doesn't extract only the user portion of an email address, even though the
help text says it does).
The new 'emailuser' filter uses the new util.emailuser(text) function which,
instead, does exactly that.
The help text on the 'user' filter has been modified accordingly.