Define Minimal YAML Data File and Schema

[sgrottel]

All data is to be stored in one single file.
Yaml is the currently hottest contender for the file format.
It's standard format and human-readability allow for easy sharing between applications and for text-editor-based self host.

Define storage and schema of the file.

Do not integrate all planned features yet!
Only add the bare minimum needed features to be useful.
Self-host as todo file (without apps) to see if it works.

[sgrottel]

• Card id options still missing