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.
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.
