devapp: further preparation for running on GKE

+ Remove superfluous check for PORT env var since only the flag
  will be used.
+ Remove HTML being written to the page before escaping step,
  causing jank.
+ Grab the GitHub token from GCE metadata if it’s available.

Updates golang/go#20691

Change-Id: I10fbc163ee91907ef0b843c823f40fd87a62f476
Reviewed-on: https://go-review.googlesource.com/46210
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed