add custom exception

- altered db schema to delete subscriptions along with topics
- extracted logic for creating and deleting topics into separate functions
- added functions to list topics and subscriptions
- delete topic and corresponding subscription if same topic is added with different token
- added more error messages and reactions
- added handling for authentication errors (subscription and topic are removed)
- inform all subscribed rooms when subscription is cancelled due to an action in another room
- added documentation for methods
- code style

ntfy/bot.py

@@ -21,8 +21,7 @@ from .exceptions import SubscriptionError
 
 async def build_notice(html_content) -> TextMessageEventContent:
     """
-    Build a notice message with optional arguments. The html_content can contain
-    placeholders like %s which will be replaced by the arguments.
+    Build a notice message.
     """
     text_content = await parse_html(html_content.strip())
     return TextMessageEventContent(