godoc: show relative file names without leading '/' (per r's request)

godoc: show relative file names without leading '/' (per r's request)

- change the various url-xxx formatters to return a relative URL path
- make the leading '/' for URLs explicit in the template
- on the way change some |html formatters to |html-esc
  (html should only be used for formatting AST nodes)

R=rsc, r
CC=golang-dev
https://golang.org/cl/740041