Replace usages of <xsl:copy-of> with <xsl:value-of> for outputting values to HTML

This prevents XML elements like <dim:field> from appearing in the generated HTML for data package and data file pages.  Previously they wrapped around titles, descriptions, download counts, etc.

These xml elements were usually ignored by web browsers, so there wasn't any visual problem.  The browser didn't render the tag and the value inside the element would still appear in the browser.  However, DOM traversal with tools such as jQuery were choking on bad markup - see https://github.com/hypothesis/h/issues/379