Fix EZP-21054: Anchor tag is not transformed by xsl

JIRA issue: https://jira.ez.no/browse/EZP-21054

# Description

the `anchor` tag is not handled by our XSLT stylesheets. As a result, when a XmlText field contains an `anchor` element, it is rendered without any transformation, and of course this is not valid in HTML5.

While at it, I also added unit tests for the Html5 converter.

# Tests

manual tests + unit tests