[WIP] graphql schema refactor

[stefanprobst]

See #4261

[stefanprobst]

@freiksenet Thanks for your comments! I think I should start by saying that I have been working on this branch as a hobby learning project, and literally the first thing I did was delete everything in the schema/ folder and start from scratch. So it's probably best to not base the schema refactor off of this, but view it more as a working prototype to take some ideas from, especially if we want to keep this as minimally invasive as possible.
I'm really motivated to help with that (and also appreciate feedback) but let's close this and do it properly?

As for the rest, comments inline.

[freiksenet]

@stefanprobst Sounds good :) I think the minimal PR would have graphql-compose backed schema, addTypeDefs and addResolvers functions and maybe the model layer. I probably won't have much time to work on it next week, but I can start or help with it after that. Looking forward to the call!