Update user session unit template.

While most folks will be using the derivative from user-session-units,
I'm updating this one to reflect some of the fixes and things to note
about user sessions:

- cgroup should be set with "%u" - username instead of %I
- set dbus path with %U explicitly too
- hint to folks that wish to use MEM_CG features in user sessions
- allow unit to be enabled for instances with systemctl enable