y-x41

Results 2 issues of y-x41

Hey! This PR adds support for performing the attack enabled by CVE-2021-34600. Details on how this attack works can be found on [our blog](https://x41-dsec.de/lab/blog/telenot-complex-insecure-keygen/). This isn't 100% complete, yet, but...

It's currently only possible to set/unset the read-only permission when sharing a calendar with someone. It'd be nice to have the option to modify that after the fact. For example...