Store IDs for access rather than usernames

**Is your feature request related to a problem? Please describe.**

When a user with permissions for commands changes their username on Twitch, they lose access to commands.

**Describe the solution you'd like**

Instead of storing usernames in the config file, store user IDs. As such, also update all the message processing logic (including the API integration) to pass the user ID instead.

**Describe alternatives you've considered**

N/A

**Additional context**

In the config file where we now have IDs, please link to a tool that'll allow anyone to quickly convert a user ID to a username, and vice versa.