godoc: replace explicit escaping with htmltemplate

godoc: replace explicit escaping with htmltemplate

I tested this by running

godoc --http=:8081 --html

in a fresh client and running the equivalent with port 8080 in
a patched client, running the script below and inspecting the result.

for path in '/' '/pkg/' '/cmd/' '/doc/go_spec.html' \
    '/doc/install.html' '/doc/go_tutorial.html' \
    '/doc/effective_go.html' '/doc/go_faq.html' \
    '/doc/docs.html' '/error.html<script>alert(42)</script>' \
    '/pkg/exp/template/html/' \
    '/src/pkg/exp/template/html/content.go?s=238:1880'; do
  echo $path
  diff -u <(curl "http://localhost:8081$path" \
            | perl -pe 's/\d+\+(. Except as noted)/<rev>+$1/') \
          <(curl "http://localhost:8080$path" \
            | perl -pe 's/\d+&#43;(. Except as noted)/<rev>+$1/')
  echo
  echo
done

The perl substitution smooths over differences in revision numbers and
in the escaping of '+'.
The remaining differences are elided comments.