Rewrite as a set of GitHub actions

[freaktechnik]

Not only would this make hosting much simpler, I believe it would make customizing it much more accessible, too. The idea from #77 kind of touches on this idea, but I think actions are better suited for what this tries to do, at least for the processing part, not necessarily for the incoming side. I'd have to look at github actions closer to figure out if they can be the incoming side, too.

[MichaelKohler]

As far as I know issue events can be triggers as well. I like this idea :)

[freaktechnik]

Yeah, but turning external triggers into issues would still likely require an external service.

[MichaelKohler]

I don't have all the external triggers in mind, but the ical could for example be checked on a cron trigger.

[freaktechnik]

Ah right, since all the incoming stuff is currently based on polling it could very well be done using a scheduled workflow.