Keep track of guild states.

We don't currently bridge anything, but this just keeps everything in sync
with the discord server.

database/migrations/07-guilds.sql

@@ -0,0 +1,7 @@
+CREATE TABLE guild (
+	discord_id TEXT NOT NULL,
+	guild_id TEXT NOT NULL,
+	guild_name TEXT NOT NULL,
+	bridge INTEGER(1) DEFAULT FALSE,
+	PRIMARY KEY(discord_id, guild_id)
+);