Implement AutoCrypt

[BjarniRunar]

The AutoCrypt (https://autocrypt.org/en/latest/level1.html) model is so similar to ours, that we should just implement it and play nice with others.

Relates to #1725 and #733.

Tasks:

• [x] Allow users to select Autocrypt as the key exchange mechanism per account
• [x] Add Autocrypt headers to outgoing mail
• [x] Add Autocrypt-Gossip headers to outgoing mail
• [x] Create Autocrypt-compatible keys by default
• [x] Parse incoming Autocrypt headers
• [x] Parse incoming Autocrypt-Gossip headers
• [x] Add Autocrypt information to the search index
• [x] Make it possible to import keys from Autocrypt and Autocrypt-Gossip headers
• [x] Maintain an Autocrypt Level 1 state database
• [x] Use the AC state database as input into crypto_policy, to effect composer defaults
• [x] Auto-import Autocrypt keys as necessary on compose, if policy wants encryption
• [x] Update local keychain when Autocrypt headers provide new keys
• [ ] Add Notifications to inform users when an PGP key gets updated/rotated
• [ ] Support the Autocrypt Level 1.1 "Setup Message" mechanism

[BjarniRunar]

I merged and pushed cd2588811d24004979db14313600b1825a66ee32 a few moments ago. This implements a significant chunk of Autocrypt support. I'm adding tickboxes to the main issue text to track what is/isn't done yet.