GET /users - Condition on usernames

Adds a parameter *contain* to the request `GET /users`, to test the usernames.
For instance, if the DB contains users with the usernames *Johannes*, *Bill* and *Elton John*, then `GET /users?contain=joh` will return the *User* instances of *Johannes* and *Elton John*.

Useful for offering autocompletion when searching for Users.