Added a Vimeo EnableDNT privacy option to the Hugo config. This will enable the Vimeo 'Do Not Track' flag when either Vimeo shortcode tempalte options are used. When enabled, it will force the Vimeo player to be blocked from tracking any session data, including all cookies and stats.
Fixes#7700
The `safeHTMLAttr` function operates on a full attribute definition, not
just within the attribute value.
Docs: https://gohugo.io/functions/safehtmlattr/
For `opengraph.html`, run the whole `content` HTML attribute through
`safeHTMLAttr`. That will preserve `+` signs in formatted dates.
For `vimeo_simple.html`, `safeHTMLAttr` was in the context of an
attribute value, thus having no effect. In this case we could replace it
with `safeURL`, but since the code is coming from an API it is safer to
just let Go's template engine sanitize the value as it already does with
`provider_url`.
Fixes#5236 (no need to change Go upstream)
Related to #5246
