add basic discord name -> discord user lookup

[owobred]

Changes

Adds a mechanism to lookup discord ids given a discord username. (expected in the format shown in the client, meaning "migrated" users without a discriminator shouldn't have a #0 appended, and unmigrated users (should just be bots) are in the format username#discriminator).
Usage should basically just be calling add_user whenever you see a discord id (probably do this in a background task to prevent backing up live-elo if we get ratelimited by discord). Whenever you have a discord username just call get_user_by_name and pray.

Checklist

• My code compiles
• I have committed all the files needed to build the project (check if your file is found in .gitignore)
• If I'm introducing a new step in the build process, I have documented / automated it
• I have tested my changes (minimally with one Twitch VOD)

To-do list

• Integrate this with whatever needed it :Clueless:
• If its preferred to always include the #0 discriminator, I can change the logic to do that.