Feature/6299 new doc

* New doc struct for encryption keys https://leap.se/code/issues/6299
* Multi uid support https://leap.se/code/issues/6212
* Keep old key after upgrade https://leap.se/code/issues/6262

This comment is actually moved in the code from somewhere else in the code.

You are right that '+' seems to be not the only character valid for tags in emails:
https://en.wikipedia.org/wiki/Email_address#Address_tags
I'm wondering if '-' is not used by some email servers as a valid email character. This needs more research, I'm opening an issue for that https://leap.se/code/issues/6393

If you are ok with it I would merge this pull-req with only support for '+' as it was already in the code and I'll research about it and fix it if needs fixing.