Fix size of roles multi-select when editing user

i.e. for buefy themes

tailbone/templates/deform/select.pt

@@ -66,6 +66,7 @@
                               placeholder '(please choose)';
                               class string: form-control ${css_class or ''};
                               :multiple str(multiple).lower();
+                              native-size size;
                               style style;
                               v-model vmodel;
                               @input input_handler;">

tailbone/views/users.py

@@ -217,6 +217,8 @@ class UserView(PrincipalMasterView):
                 size = len(roles)
                 if size < 3:
                     size = 3
+                elif size > 20:
+                    size = 20
                 f.set_widget('roles', dfwidget.SelectWidget(multiple=True,
                                                             size=size,
                                                             values=role_values))