Tracing overlay: prevent mixing connector types when creating new links

The CATMAID front-end assigns types to connectors based on its links to
treenodes (or uses the default connector type), e.g. 'synaptic' or
'desmosome'. When creating new links it was possible so far to add links
to a connector that wouldn't fit to its existing links, even causing the
(non-persisted) connector type to be overridden. This is fixed now by
checking the connector types before adding a new link to a connector
that already has links. If they conflict, a warning is shown and now
link is added. Thanks to Sanja Jasek for bringing this up.

django/applications/catmaid/static/js/widgets/overlay-node.js

@@ -1501,7 +1501,8 @@
             // connected activated treenode or connectornode
             // to existing treenode or connectornode
             if (atnType === SkeletonAnnotations.TYPE_CONNECTORNODE) {
-              var atnSubType = SkeletonAnnotations.getActiveNodeSubType();
+              let atnSubType = SkeletonAnnotations.getActiveNodeSubType();
+              let connectorNode = catmaidTracingOverlay.nodes.get(atnID);
 
               // If the Alt key is pressed, we want to show a menu for
               // connector type selection.
@@ -1514,11 +1515,19 @@
                 catmaidTracingOverlay.askForConnectorType()
                   .then(function(selection) {
                     if (selection) {
-                      SkeletonAnnotations.atn.subtype = selection.value;
-                      catmaidTracingOverlay.createLink(node.id, atnID, selection.relation)
-                        .catch(CATMAID.handleError);
+                      // Don't allow link combinations that would result in a
+                      // mixed connector type.
+                      if (connectorNode.links && connectorNode.links.length > 0 &&
+                          connectorNode.subtype !== selection.value) {
+                        throw new CATMAID.Warning(`Can't mix connector types ` +
+                            `${connectorNode.subtype} and ${selection.value}`);
+                      }
+                      connectorNode.subtype = selection.value;
+                      catmaidTracingOverlay.createLink(node.id, atnID,
+                          selection.relation);
                     }
-                  });
+                  })
+                  .catch(CATMAID.handleError);
                 return;
               }
 
@@ -1778,11 +1787,19 @@
                 catmaidTracingOverlay.askForConnectorType()
                   .then(function(selection) {
                     if (selection) {
+                      // Don't allow link combinations that would result in a
+                      // mixed connector type.
+                      if (connectornode.links && connectornode.links.length > 0 &&
+                          connectornode.subtype !== selection.value) {
+                        throw new CATMAID.Warning(`Can't mix connector types ` +
+                            `${connectornode.subtype} and ${selection.value}`);
+                      }
                       connectornode.subtype = selection.value;
-                      catmaidTracingOverlay.createLink(atnID, connectornode.id, selection.relation)
-                        .catch(CATMAID.handleError);
+                      return catmaidTracingOverlay.createLink(atnID,
+                          connectornode.id, selection.relation);
                     }
-                  });
+                  })
+                  .catch(CATMAID.handleError);
                 return;
               }
               if (connectornode.subtype === CATMAID.Connectors.SUBTYPE_GAPJUNCTION_CONNECTOR) {