RESUtils.initObservers watches the real #siteTable for changes

The "sponsored link" on /r/all and large subs is currently wrapped in a `#siteTable` element. RES's siteTable watcher is listening for changes on that, rather than the actual link listings.

This change adds a guard against the advert #siteTable so that the MutationObserver/DOMNodeInserted listens to the link listings container.

Fixes #272