WIP: Do Not Merge - rewrite playground

[sit]

Spent the weekend playing around with how we might re-write the bot to be cleaner and easier to modify/test.

Here are some experiments:

• Use jest for testing
• Make a domain-only PracticeManager that is divorced from Discord events or persistence.
• Add a RoleManager sketch that might serve to allow us to add roles, cleanly separated from tracking practice time.
• Rewrite database_lib and leaderboard_fetch as persistence.js (see persistence.int.test.js for usage)

[pianiststickman]

SessionManager is now a thing, as is Persistence. RoleManager eschewed in favour of badges. Thanks for setting up jest!