Modify 404 group page query

Now uses the 'word_similarity' function (and uses it by name) and
enforces a minimum match on the data in addition to the max 3 limit.

tildes/tildes/templates/error_group_not_found.jinja2

@@ -9,7 +9,7 @@ Group not found
 {% block content %}
 
 <div class="empty">
-  <h2 class="empty-title">No group by that name</h2>
+  <h2 class="empty-title">No group named '{{ supplied_name }}'</h2>
   {% if group_suggestions %}
     <p class="empty-subtitle">
       Did you mean one of these groups instead?

tildes/tildes/views/exceptions.py

@@ -3,7 +3,7 @@
 from pyramid.httpexceptions import HTTPNotFound
 from pyramid.request import Request
 from pyramid.view import forbidden_view_config, exception_view_config
-from sqlalchemy import text
+from sqlalchemy import cast, desc, func, Text
 
 from tildes.models.group import Group
 
@@ -21,12 +21,13 @@ def forbidden(request: Request) -> dict:
 def group_not_found(request: Request) -> dict:
     """Show the user a customized 404 page for group names."""
     request.response.status_int = 404
-    supplied = request.matchdict.get("group_path")
+    supplied_name = request.matchdict.get("group_path")
+    # the 'word_similarity' function here is from the 'pg_trgm' extension
     group_suggestions = (
-        request.db_session.query(Group)
-        .order_by(text("ltree2text(path) <<-> :supplied"))
-        .params(supplied=supplied)
+        request.query(Group)
+        .filter(func.word_similarity(cast(Group.path, Text), supplied_name) > 0)
+        .order_by(desc(func.word_similarity(cast(Group.path, Text), supplied_name)))
         .limit(3)
         .all()
     )
-    return {"group_suggestions": group_suggestions}
+    return {"group_suggestions": group_suggestions, "supplied_name": supplied_name}