Properly cleanup on code intelligence unsubscribe event

Created by: erzhtor

In current behavior, when changing Sourcegraph URL, browser-extension re-initializes itself in all existing/open pages. It works fine if a new URL works for the page.

However, if the new Sourcegraph URL/instance does not support page/code host, it just de-taches all JS logic, but the UI part is left as is (with all buttons and popups) until refreshing the page. So it would be good in terms of UX to properly cleanup UI elements as well.