Rework all the stuff

[christopher-dG]

The current GitHub Action is more like a proof of concept than something I'd actually want to see in production. In terms of implementation, most of the functions have now been moved to a Repo class that holds state, instead of global state being determined by the environment variables set by GitHub Actions. This is quite a bit nicer IMO, and with it I'd say we can move forward with #34, and #39 after a testing period (and probably some integration tests).

Closes #40 (only a GitHub release is created by default, with the option to put annotated tags underneath for yourself via the dispatch input)
Closes #37 (no more GPG)
Closes #36 (no more inputs with dashes)
Closes #35 (this works now)

There is a notable lack of changelog tests, this is because I want to rework that section soon too to allow for more customization. For now, I can say that it works on my machine 🙃.

Also to note is that support for self-hosted GitHub is gone. I'll add it back if someone asks for it but I do not know of any organizations using Julia that hosts GitHub on-prem.

I'll probably merge tomorrow unless there's something awful that someone finds.

edit: not sure why CI is failing, it works locally (although it's on 3.8 vs 3.7).
Hm, this is a Python 3.7 vs 3.8 issue. 3.8 works as I would expect, 3.7 does not.