Serve mock index.html and window.context

Created by: felixfbecker

We currently need to record a HAR for every path, despite those serving the same index.html with only minor differences like the <title>. This could be annoying to record for pages requiring backend state, like the index.html for a campaigns details page. We also need the ability to stub window.context, in a type-safe way. This makes us always serve a mock index.html, which removes the need for recording HARs on the webapp. HARs will still be used for the browser extension though.

Also added NODE_NO_WARNINGS for now because Polly causes a lot of uncaughtException warnings and web-ext's transitive dependency of an old version of shelljs causes circular dependency warnings. I don't like silencing warnings, but they were extremely verbose (make the test output barely readable), and are outside of our control. We have an ESLint rule to catch uncaught exceptions.