Overhaul usage of sourcegraph URL in browser extension

Created by: felixfbecker

At the moment, multiple ways are used to access the Sourcegraph URL:

• window.SOURCEGRAPH_URL
• localStorage.getItem('SOURCEGRAPH_URL')
• storage.getSync(({ sourcegraphURL }) => ...)
• import { sourcegraphURL } from ...

We need to define and use a single source of truth and also react to its changes everywhere (e.g. with an Observable)