[feat] Display encryption status of email addresses in compose window

When composing an email, display the encryption status for each address. If the public key for a given address is found, show the "lock" sign (encrypted) in front of it otherwise show the "unlocked" sign (unencrypted). This status is fetched using the keymanager.

Note: I realize Pixelated follows a different approach for showing the encryption status where it adds the lock icon next to the email address itself. For Thunderbird, we are doing it on a per-row basis instead. This makes it look more natural for a Thunderbird user and also the implementation is easier :)

I think this is fine for now as it serves our purpose; we can revisit this later if we want to. (Also, it's just displaying the encryption status so maybe I am overthinking it.)

Obligatory screenshot: