New EnvUser

Add:
 - the EnvUser doc
 - support for setting the last login time for the EnvUser
 - factory methods for adding the envuser

Create the Admin EnvUser at bootstrap time.
Is there a chance of running into problems with createdBy not being a valid user id? Maybe this should be "admin"? Or, even better, make User and CreatedBy in EnvUserParams be *state.User - that way valid users can be inserted, or created on the fly.