change group owner from 'manager' to 'user'

This will allow privacy management to use existing code.
Some template updates also are for rationalising how
groups are created and edited.

bookwyrm/templates/groups/created_text.html

@@ -1,6 +1,6 @@
 {% load i18n %}
 {% spaceless %}
 
-{% blocktrans with username=group.manager.display_name path=group.manager.local_path %}Managed by <a href="{{ path }}">{{ username }}</a>{% endblocktrans %}
+{% blocktrans with username=group.user.display_name path=group.user.local_path %}Managed by <a href="{{ path }}">{{ username }}</a>{% endblocktrans %}
 
 {% endspaceless %}