Add a Discord rank syncing endpoint

[marcdif]

This endpoint will be called whenever a Minecraft user's rank changes, and their display role on Palace's discord needs to be changed. The body should contain the minecraftUUID and the rank they're changing to, as a lowercase string.

If there is an error - the UUID doesn't exist, the rank doesn't exist, etc. - return a message that explains the issue.
If it succeds, return success: true.