Deal with prefer encryption correctly

• Read https://autocrypt.org/level1.html
• Discussion with Oliver
• by default, prefEnc should be true and only false if the owner of the address has set it to false
• depending on prefEnc the sendView should send it encrypted or not
• there should probably be some information to the user about that
• Can a user change the own prefEnc state? What is the default state?
• A user should be able to enforce encryption
• A user should be able to set a default encryption preference: Enforce encryption every time or following prefEnc...