Fix session.domain_context_name existence check
In DEBUG mode this check currently causes a VariableDoesNotExist error.
Fix by using `if x in y` rather than `if x.y`.
{% block page_header %}
<div class='page-header'>
<h1>
- {% if request.session.domain_context_name %}
+ {% if 'domain_context_name' in request.session %}
<em>{% blocktrans with context_name=request.session.domain_context_name %}{{ context_name }}:{% endblocktrans %}</em>
{% endif %}
{{ title }}
