Notable refactoring:
- Use stdlib "context" in place of "golang.org/x/net/context"
- Go-kit no longer wraps errors, so we remove the unwrap in transport_error.go
- Use MakeHandler when setting up endpoint tests (fixes test bug caught during
this refactoring)
Closes#1411.
* Defining a concrete type for session tokens
* More rightish vc.IsLoggedIn()
* using type conversion instead of a method call
* include sessions in test viewer contexts
