Having spoken to @tfm I really don't think these methods should be here on the interface, given that 4 should know nothing about slugs. Alternatives:

1.  put the methods directly the MongoAppStore and refer to that in MongoUserStore
1.  migrate 3 to have longs _and_ slug app refs
1.  migrate 3 to only have long app refs

Thoughts? Cc @tfm @oli-hall