html/template: add srcset content type

html/template: add srcset content type

Srcset is largely the same as a URL, but is escaped in URL contexts.
Inside a srcset attribute, URLs have their commas percent-escaped to
avoid having the URL be interpreted as multiple URLs.  Srcset is placed
in a srcset attribute literally.

Fixes #17441

Change-Id: I676b544784c7e54954ddb91eeff242cab25d02c4
Reviewed-on: https://go-review.googlesource.com/38324
Reviewed-by: Kunpei Sakai <namusyaka@gmail.com>
Reviewed-by: Mike Samuel <mikesamuel@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>