Follow up Heimdall-scheduled Zoom calls: v2

[kb0rg]

The MVP version of this functionality was addressed in Issue #46
This is a follow-up to address some of the notes from PR #56

The next iteration should:

• refactor to use async/ await instead of Promises
• refactor getMeetingDetails and pull it intolib.zoom.Session
• refactor meetingEndedPromise to use a type with constant strings