Remove ReplyToken#new_reply_token
Always generate new reply token when creating User
Make intent of FillInReplyTokensTransition clearer
Create ReplyToken.generate to save key strokes
Use double quotes for mister @houndci
Add 'not null' constraint and index to reply token
Remove FillInReplyTokensTransition

Removing this because we now have a not null constraint. It is
impossible to test this transition.