Handle key import via Autocryptsetup-Message

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