add support for html meta tag

``

When dynamically setting the content attribute of a meta tag I get the following error.

```Critical: frontend.NewServer: NewServer(...): s.renderErrorPage(http.StatusInternalServerError, nil): html/template:base.tmpl:13:36: cannot escape action {{.Description}}: actions must not occur in the "content" attribute value context of a "meta" element```

This tag is useful for setting the description of a page in search results or link summary text for applications like slack and I think should be considered safe.

golang/go#40752