This will be used to show the "Run" button for code snippets in go.talks only for Go when running on App Engine.
R=adg, r, iant
CC=golang-dev
https://golang.org/cl/21930047
Also update style.css to hide outline of editable text areas and apply
correct styles to line numbers in non-playground snippets.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13946043
Godoc depends on go.talks/pkg/present by way of go.tools/pkg/blog.
Better to keep all godoc dependencies in one place.
R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/13656047
