Allow linking to topics without any title slug

This is a bit risky because of potential conflicts with other routes
(like "~group_name/new_topic" if "new_topic" was a valid ID36), but it
was being used a little before so it's probably better to have
available.

tildes/tildes/routes.py

@@ -37,6 +37,7 @@ def includeme(config: Configurator) -> None:
         config.add_route("group_topics", "/topics", factory=group_by_path)
 
         config.add_route("topic", "/{topic_id36}/{title}", factory=topic_by_id36)
+        config.add_route("topic_no_title", "/{topic_id36}", factory=topic_by_id36)
 
     config.add_route("user", "/user/{username}", factory=user_by_username)
     with config.route_prefix_context("/user/{username}"):

tildes/tildes/views/topic.py

@@ -255,6 +255,7 @@ def get_new_topic_form(request: Request) -> dict:
 
 
 @view_config(route_name="topic", renderer="topic.jinja2")
+@view_config(route_name="topic_no_title", renderer="topic.jinja2")
 @use_kwargs({"comment_order": Enum(CommentTreeSortOption, missing="relevance")})
 def get_topic(request: Request, comment_order: CommentTreeSortOption) -> dict:
     """View a single topic."""