fixed _getTerm function, which was not properly checking access rights
all terms are stored under 'category' which is a little confusing, since
other taxonomies are called differently.. this needs to be cleaned at a
later point in time, for now - this is the easiest way
The disadvantage currently is, that group associations with terms are
stored on a term basis only. If an identical term is used for different
taxonomies, the group access will be identical. So each group needs to
be access rights for term-taxonomy combinations