ad hoc backend user creation

[ztsai]

• when resolving user with appId and userId, aka backend user:

  1. do a term query on appId and appUserId (could also query on id to be appId_sha(userId) but there might be collision)
  2. if the user doesn't exist, create a new user with id being appId_sha(userId)[iii] and randomly generated name and avatar.
  3. in case of collision, generate another id somehow (since collision is unlikely, collision prevention mechanism will be tracked separately in user id collision prevention #222 )

• unit tests