Slash commands and updates to codebase

[Fluxticks]

Worked on this update while bored at work so not fussed if this PR is left open indefinitely. Changes to almost every aspect of the bot, improves user interaction experience (via slash commands) and codebase readability. Docker related items may need to be updated as I have not touched Docker or the config files while working on this.

Features of update:

• Slash Commands: No longer relies on users knowing commands / command prefix and handles a lot of input validation
• Some unused extensions have not been implemented for various reasons.
  • Tweet forwarding needs a Twitter V2 API key for and it's relevant "Bearer Token" which I cannot acquire.
  • TwitchCog "go-live" notifications, usually there is an announcement pre-stream and thus making the notification somewhat redundant.
  • VotingCog, similar to PingableRoles it sees little to no usage and would require a decent amount of work to implement.
• Functionality of other previous extensions have been either kept or improved.
• Codebase restructured to improve development and readability.