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.
If you change st.EnvironmentUser to take a tag, then you could just go:
```go
eUser, err := st.EnvironmentUser(names.NewUserTag(uDoc.Name))
```