Add GPG per recipient rules

Alice and Bob want to have encrypted communication and already have each other's public GPG keys.
Alice is using Thunderbird with Enigmail and has a rule to always encrypt messages to Bob.

When Bob replies to the message using K9, or starts a new thread to Alice, he has to remember each time to enable encryption on each message, which is error prone.

K9 mail should have an option to say that for a specific e-mail address, to default to enabling encryption with that key. That way, Bob's messages would be encrypted, unless otherwise specified. There are also complications for how to handle e-mails will multiple recipients, but those are less common.

Related:
https://github.com/k9mail/k-9/issues/3451

Expected behavior

Bob adds a per-recipient rule, and messages to Alice are encrypted.

Actual behavior

Bob has to click encrypt for every new message composed, leading to some messages not being encrypted.

Steps to reproduce

1. Alice and Bob exchange GPG public keys
2. (Optional) Alice sends encrypted message to Bob
3. Bob composes e-mail to Alice

Environment

(Not available)

The issue does not identify files, tests, or an entry point. First clarify how per-recipient rules should interact with multiple recipients and existing encryption behavior, then define tests for default encryption when composing messages to a matching recipient.