Replaces the UI endpoints for creating and modifying labels. These were removed
in #1686 because we thought we were killing the UI.
Now labels can be created and edited in the UI again.
With the UI, deleting by ID made sense. With fleetctl, we now want to delete
by name. Transition only the methods used for spec related entities, as others
will be removed soon.
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.
