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/snippets/add_to_group_button.html

@@ -1,5 +1,5 @@
 {% load i18n %}
-{% if request.user == user or not request.user == group.manager or not request.user.is_authenticated %}
+{% if request.user == user or not request.user == group.user or not request.user.is_authenticated %}
 <!-- TODO: blocking is irrelevant here, but ...is it? -->
 {% elif user in request.user.blocks.all %}
 {% include 'snippets/block_button.html' with blocks=True %}