singlepage.html: validation messages need to be HTML-escaped

[vaitkus]

Validation messages output in the webpages of individual developers are currently not properly escaped, most likely to accommodate replacement of \n with <br />:

providers-dashboard/make_ghpages/mod/templates/singlepage.html

Lines 109 to 113 in 01223ff

	{% for message in index_metadb.subdb_validation[subdb.attributes.base_url]['failure_messages'] %}
	{% set bad_url = message[0].split(" - ")[0] %}
	❌ <a href="{{ bad_url }}">{{ bad_url | safe }}</a><br /><br />
	<p>{{ message[1].replace("\n", "<br />") | safe }}</p><br />
	{% endfor %}

As a result, symbols like "<" are not escaped (for example in the ResponseError: Request to 'structures?filter=nperiodic_dimensions < 3' returned HTTP status code 500. message).