make support/3.x branch green again; allow it to co-exist with realm SDK in same JS context

[mpobrien]

summary of changes:

• updated the stitch server lib setup to be correct
• i got rid of fetch-everywhere, because it pollutes the global namespace. This means that if you import it in the same context as some other module that relies on fetch, it might produce unexpected behavior (spoiler: this happens when you import both the Stitch and Realm SDKs).
• instead of fetch-everywhere we just use node-fetch (which is what fetch-everywhere is wrapping, anyway).
• tests that involve mocking the behavior of fetch now work by either using fetch-mock or sinon as before, but explicitly populating it in global for only the lifetime of the test.
• fixed a bunch of tests including one that seemed to have broken setup/teardown for local storage
• also fixed https://github.com/10gen/stitch/tree/050e91953eccd5d8a19625bdd3ec8ff379d153eb in the server repo which was causing a bunch of these tests to fail

[mpobrien]

Update: I tried this in the UI as well as running the tests, seems to be working OK everywhere.