Refactor events.js into Recorder and Events class

[bensheldon]

• Removes some of the unused application stuff from Demo (like PWA)
• Uses JS classes
• Separates out Start/Stop for Pause/Unpause to better differentiate navigation events (when it's necessary to restart the recorder) from visibility events (when we just want to stop/start any timers).
• Separates the Recorder class, which manages rrweb and hooks into the lifecycle events, and the Events class which handles storing events and transmitting them to the backend.
• This Chrome article about Page Lifecycle API is helpful: https://developer.chrome.com/docs/web-platform/page-lifecycle-api
• I could probably use arrow functions instead of bind but I had written them that way initially and I'll wait until some sort of compatibility linting is in place to really worry