Fix creation of card links

Without this fix, orphaned card links are created and therefore this leads to
problems as described in https://github.com/wekan/wekan/issues/2785.

client/components/lists/listBody.js

@@ -658,10 +658,7 @@ BlazeComponent.extendComponent({
               _id = element.copy(this.boardId, this.swimlaneId, this.listId);
               // 1.B Linked card
             } else {
-              delete element._id;
-              element.type = 'cardType-linkedCard';
-              element.linkedId = element.linkedId || element._id;
-              _id = Cards.insert(element);
+              _id = element.link(this.boardId, this.swimlaneId, this.listId);
             }
             Filter.addException(_id);
             // List insertion

models/cards.js

@@ -428,6 +428,21 @@ Cards.helpers({
     return _id;
   },
 
+  link(boardId, swimlaneId, listId) {
+    // TODO is there a better method to create a deepcopy?
+    linkCard = JSON.parse(JSON.stringify(this));
+    // TODO is this how it is meant to be?
+    linkCard.linkedId = linkCard.linkedId || linkCard._id;
+    linkCard.boardId = boardId;
+    linkCard.swimlaneId = swimlaneId;
+    linkCard.listId = listId;
+    linkCard.type = 'cardType-linkedCard';
+    delete linkCard._id;
+    // TODO shall we copy the labels for a linked card?!
+    delete linkCard.labelIds;
+    return Cards.insert(linkCard);
+  },
+
   list() {
     return Lists.findOne(this.listId);
   },