Stub MP3 downloading for e2e

[davefollett]

✨ New feature description

When the web app tries to fetch MP3s from the network we should use cypress to intercept the call and return a mp3 from a cypress fixture.

➕ Additional context

The solution is partially coded but does not work. The following error is encountered from offlineEpisodeHook.js

Failed to execute 'put' on 'Cache': parameter 2 is not of type 'Response'

To view the error, do the following:

1. Remove .skip from L25 of the audio.spec.js test
2. Start the web app: npm start
3. Start cypress in interactive mode: npm run cypress:open
4. Run just the audio test by clicking it on the cypress window

It seems like this issue is the real response has a proto of this: __proto__ : Response but my stubbed response proto is: __proto__ : Object.