Internal Update, unnotify Command

What's new?

• New error message when using thread-only commands outside of threads.
• ?unnotify, ability to undo ?notify.
• ?notify and ?subscribe now accepts other users.

Changes

This update contains mostly internal changes.

• Implemented support for the new discord.py v1.1.1.
• Improved help text for most commands.
• Completely revamped help command, few user changes.
• Removed abc (internal).