Clean up request-handler, github-auth, and analytics; upgrade to Mocha 4 (#1142)

- Add tests to request-handler to prepare for some tweaks to caching for #820
- Clean up code in request-handler: renames, DRY, arrows, imports
- Allow for clean shutdown of `setInterval` code. This requires the ability to cancel autosaving.
- Upgrade to Mocha 4, and clean up so the process exits on its own (see mochajs/mocha#3044)
- Better encapsulate analytics