Handle key import via Autocryptsetup-Message

wieseoli
2020-02-21T09:58:09Z

We should consider how to import a key via autocrypt setup message. (Mailhandler and Autocrypt class)
Option 1: User opens a setup message mail and likes to import the key.
Option 2: Import a key (in key management) offers a list of possible keys. (similar to itunes key import)

* [ ] Read https://autocrypt.org/level1.html
* [ ] Refactor Mailhandler and Autocrypt class
* [ ] Add missing logic
* [ ] Create paper prototype
* [ ] Add UI