Use "%" for user wildcard search instead of "*"

Recent update of the accounts code exposed this incorrect use of user
wildcard search

https://github.com/openstax/accounts/commit/70aaab5f0247642d9aadb6bb96c90dd05613bcfb
Merge pull request #9 from Connexions/user-wildcard-search

Use "%" for user wildcard search instead of "*"