Replaced Taxonomy is_optional with is_multiple

where the code reads better with is_multiple

django/website/data_layer/models.py

@@ -44,7 +44,7 @@ class Message(DataLayerModel):
         if not all(t.taxonomy == taxonomy for t in terms):
             raise ItemTermException("Terms cannot be applied from different taxonomies")
 
-        if taxonomy.is_optional:
+        if not taxonomy.is_multiple:
             if len(terms) > 1:
                 message = "Taxonomy '%s' does not support multiple terms" % taxonomy
                 raise ItemTermException(message)

django/website/data_layer/tests/item_tests.py

@@ -138,7 +138,7 @@ def test_apply_terms_adds_term_for_tags():
     taxonomy = TaxonomyFactory(multiplicity='multiple')
     term1 = TermFactory(taxonomy=taxonomy)
     term2 = TermFactory(taxonomy=taxonomy)
-    assert not taxonomy.is_optional
+    assert taxonomy.is_multiple
 
     item.apply_terms(term1)
     assert list(item.terms.all()) == [term1]