group membership invitations

- fix display of group information on user and group pages
- send, receive, accept and reject invitations

bookwyrm/templates/snippets/join_invitation_buttons.html

@@ -0,0 +1,16 @@
+{% load i18n %}
+{% load bookwyrm_group_tags %}
+{% if group|is_invited:request.user %}
+<div class="field is-grouped">
+    <form action="/accept-group-invitation/" method="POST">
+        {% csrf_token %}
+        <input type="hidden" name="group" value="{{ group.id }}">
+        <button class="button is-link is-small" type="submit">{% trans "Accept" %}</button>
+    </form>
+    <form action="/reject-group-invitation/" method="POST">
+        {% csrf_token %}
+        <input type="hidden" name="group" value="{{ group.id }}">
+        <button class="button is-danger is-light is-small" type="submit" class="warning">{% trans "Delete" %}</button>
+    </form>
+</div>
+{% endif %}