Add GitHub issue handler

[nakajimayoshi]

Add a handler which leverages the GitHub API to sync issues to discord.

Features:
- Open new thread when new issue is opened
- Auto closing of threads on issue close
- Update threads when new message is posted

[martinheidegger]

This can/could be done using github actions! (at least in the gh → discord direction) the other way is more difficult.

[nakajimayoshi]

The broadcasting will be the easy part as that is the main responsibility of the GitHub API, since various discord actions would occur in response to different GitHub events (closing of threads, updating, opening) it'd be easier to put all the logic in one "handler" on the bot