Separate GitHub, Slack, and main

[MartinKolarik]

• The GitHub app is currently nested in the Slack app
• The Discord app, although in its own package (and currently disabled), is being loaded from the Slack app to run

Refactor this so that there are three independent Discord, GitHub, and Slack packages, and then possibly a "main" package, that loads and starts all three.

[radulucut]

@MartinKolarik are you thinking having these running on a different port each, behind nginx or just one node process?

[MartinKolarik]

For now, let's just have each app export its own middleware that gets mounted on the main package. So 1 process but clear structure.