Explicitly adding lodash as dependency of the web interface.

It was missing before, although used in at least one line of code. The
reason why it worked before is because it was drawn it transitively
through other libraries depending on it.

graylog2-web-interface/package.json

@@ -30,6 +30,7 @@
     "immutable": "^3.7.5",
     "javascript-natural-sort": "^0.7.1",
     "jquery-ui": "1.12.x",
+    "lodash": "^4.17.4",
     "markdown": "^0.5.0",
     "md5": "^2.0.0",
     "moment-duration-format": "1.3.0",