Add basic switch model and migrate fronter command

src/pluralkit/switch.py

@@ -0,0 +1,11 @@
+from collections import namedtuple
+
+from typing import List
+
+from pluralkit import db
+from pluralkit.member import Member
+
+
+class Switch(namedtuple("Switch", ["id", "system", "timestamp", "members"])):
+    async def fetch_members(self, conn) -> List[Member]:
+        return await db.get_members(conn, self.members)