XP System

[linschris]

Hello, I've done a bit of work, just finishing up the XP System. I split up the commits for readability, so feel free to squash/combine/modify them how you please.

[zeroclutch]

Also, please make this PR for staging

[zeroclutch]

Nice changes, we're still running into an issue though. When we try to updateXP, we get this error:

"type": "MongoInvalidArgumentError",
"message": "Update document requires atomic operators",

Take a look at the MongoDB operators that are used in the second argument in the updateOne method, I think we'll need to use those rather than writing {xp, level} directly.

[linschris]

Hi, just a question @zeroclutch:

• Do we need to worry about like calling updateXP with values like Infinity, undefined, etc.?