docs/appengine: move sendgrid sample, clean up
docs/appengine: move sendgrid sample, clean up

Change-Id: I19a3af3da31748737b3347274527aeca1d8a94c5

campoy/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,campoy/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,danicat/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,campoy/golang-samples,GoogleCloudPlatform/golang-samples