Add the EnvUser with tests. Create an Environment user during bootstrap

Added the EnvUser doc

Support for setting the last login time for the EnvUser

Added a factory for adding the envuser

Admin EnvUser gets added at bootstrap time

Removed a couple of attributes from the upgrade package that no longer needed to be there

@fwereade @howbazaar. As user and createdBy are expected to be users it might make sense to have them passed in a names.UnitTag (but stored as strings internally). This would at least allow some validation that they are valid upon creating of the EnvUser. What do you think?