Mojang/Minecraft Auth

It'd be cool if we could call out to minecraft and decode minecraft usernames. I'm not sure if this is offically supported, and I can't tell if one is actually supposed to use this, so there's probably some more research needed to see if this is even something that should be done.

Anyways [this outlines an auth flow](https://wiki.vg/Authentication) but it isn't oauth compliant.

Start by reviewing the linked wiki.vg Authentication flow and the existing Flask endpoint for linking external accounts. Verify whether Mojang officially supports the proposed username decoding and whether the flow can fit the project's account-linking model; done means a documented decision and, if approved, a defined implementation scope.

索引モデルが issue の本文から書いたものです。