User management: disable Authentication tab content for non admin users (#718)

* User management: first commit

* User management: remove wrong setState

* User management: remove debug log

* lint

* format

* cr fixes

* refactor from services to middleware

* remove userService.js

server/middleware/requireAdmin.js

@@ -0,0 +1,9 @@
+module.exports = (req, res, next) => {
+  if (req.user == null || !req.user.isAdmin) {
+    return res
+      .status(403)
+      .json({message: 'User is not admin.'})
+      .send();
+  }
+  next();
+};