v3: define migration spec (add files)

migrations/api_v3/groups.js

@@ -0,0 +1,17 @@
+/*
+  name is required
+  leader is required
+  type is required
+  privacy is required
+  leaderOnly.challenges is required
+  members are not stored anymore
+  invites are not stored anymore
+  challenges are not stored anymore
+  balance > 0
+  memberCount must be checked
+  challengeCount must be checked
+  quest.leader must be present (default to party leader)
+  quest.key must be valid (otherwise remove)
+
+  tavern id and leader must be updated
+*/