(CE-1143) Load JSSnippets in Monobook

In PR #5237, loading of JSSnippets was moved from a hook to the
AssetsManager config, but wasn't added to load in monobook_js as well.
This caused a number of JS issues in Monobook for extensions that made
use of JSSnippets, so we just need to add JSSnippets to monobook_js.

This fixes the following tickets:
* [CE-1143](https://wikia-inc.atlassian.net/browse/CE-1143): Tabber is broken in the Monobook skin
* [CE-1144](https://wikia-inc.atlassian.net/browse/CE-1144): RSS feeds still broken in Monobook
* [CE-1147](https://wikia-inc.atlassian.net/browse/CE-1147): Polls in Monobook are not working

/cc @lizlux @Wikia/community-engineering