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/members.html

@@ -38,7 +38,7 @@
           <span title="{{ member.display_name }}" class="is-block is-6 has-text-weight-bold">{{ member.display_name|truncatechars:10 }}</span>
           <span title="@{{ member|username }}" class="is-block pb-3">@{{ member|username|truncatechars:8 }}</span>
       </a>
-      {% if group.manager == member %}
+      {% if group.user == member %}
       <span class="icon icon-star-full" title="Manager">
         <span class="is-sr-only">Manager</span>
       </span>