Use blocktrans in heading of register_members_table

aleksis/apps/alsijil/templates/alsijil/partials/register_members_table.html

-<h4>{% trans 'Persons in Group' %} {{ group.name }}</h4>
+<h4>{% blocktrans with group=group.name $}Persons in Group {{ group }}{% endblocktrans %}</h4>
 
 <table id="persons">
   <thead>