Revert "Update dependency @vaadin/vaadin-context-menu to v5 (#2381)" (#2414)

This reverts commit 158861f.

Author: KyleJu

webapp/package-lock.json

@@ -69,7 +69,7 @@
     "@polymer/paper-toggle-button": "3.0.1",
     "@polymer/paper-tooltip": "3.0.1",
     "@polymer/polymer": "3.4.1",
-    "@vaadin/vaadin-context-menu": "5.0.0",
+    "@vaadin/vaadin-context-menu": "4.5.0",
     "@vaadin/vaadin-date-picker": "4.4.1",
     "@vaadin/vaadin-grid": "5.7.7",
     "@webcomponents/webcomponentsjs": "2.5.0",