fix wrongly escaped HTML in <noscript> tags

Currently on master (and on meta.discourse.org), in `<noscript>` tags, there is:

    <p><a href="/popular?page=1">next page &amp;rarr;</a></p>
    <p>Powered by &lt;a href=&quot;http://www.discourse.org&quot;&gt;Discourse&lt;/a&gt;, best viewed with JavaScript enabled</p>

which is a wrongly escaped HTML. This happened because of translations, which escape HTML by default. This commit fixes it by inserting the arrow directly as a unicode character and changing the name of `powered_by` into `powered_by_html`.