No caching of default plan

[ArtrenH]

The caching system should check for each school, if a default plan exists (there should be an endpoint for that) and cache it if it is existent.

[Belissimo-T]

The endpoint is https://www.stundenplan24.de/SCHOOLNR/splan/.
It is not used by the WOG.
Downloading/Parsing is also not yet implemented in https://github.com/Belissimo-T/stundenplan24-wrapper.
I'm gonna focus on #47 first.

[ArtrenH]

Will this ever be implemented (in the great weekend refactor for example)?

[ArtrenH]

Btw, Im not sure, how that was supposed to be implemented because the default plan obviously should be cached just like the other plans. The only thing, we could use this for is for getting a default date.