Fix a typo when reacting to an existing invocation

checkExistingTasks was looking up the task's `self` link by
checking for `rel=self`, which failed. I have corrected this
to `rel="self"` - `self` would have evaluated to `undefined`.

webapp/src/components/genericComponents/taskSubmitter.vue

@@ -194,7 +194,7 @@ export default {
             this.$emit("taskStarted");
             this.startPolling(
               task.id,
-              task.links.find(t => t.rel == self).href
+              task.links.find(t => t.rel == "self").href
             );
           }
         }