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/layout.html

@@ -12,7 +12,7 @@
         </p>
     </div>
     <div class="column is-narrow is-flex">
-        {% if request.user == group.manager %}
+        {% if request.user == group.user %}
             {% trans "Edit group" as button_text %}
             {% include 'snippets/toggle/open_button.html' with text=button_text icon_with_text="pencil" controls_text="edit_group" focus="edit_group_header" %}
         {% endif %}