Signin with discord

[nint8835]

Add Discord-based SSO, controlled by --use-discord-oauth on the almanac serve command.

Use requires some config to be set in almanac.toml, example is:

[discord]
client_id = "CLIENT_ID"
client_secret = "CLIENT_SECRET"
session_secret = "secret"
callback_url = "http://localhost:10000/oauth/callback"
guild_id = "GUILD_ID"