Pass correct user id in Move notification

We were passing the *requesting* user's moved_to value to the Move notification template, instead of the id of the user that they are being notified about.
Additionally, the id_to_username template tag had no fallback for if the user_id is None.

This resolves both problems and removes an unnecessary space in a template for when the logged in user made the move.

Fixes #3196

bookwyrm/templates/notifications/items/move_user.html

@@ -14,7 +14,7 @@
 
 {% block description %}
     {% if related_user_moved_to %}
-        {% id_to_username request.user.moved_to  as username %}
+        {% id_to_username related_user_moved_to as username %}
         {% blocktrans trimmed %}
         {{ related_user }} has moved to <a href="{{ related_user_moved_to }}">{{ username }}</a>
         {% endblocktrans %}

bookwyrm/templatetags/utilities.py

@@ -126,6 +126,7 @@ def id_to_username(user_id):
         value = f"{name}@{domain}"
 
         return value
+    return "a new user account"
 
 
 @register.filter(name="get_file_size")