New source creation uses Sequoia

[legoktm]

As part of our work to migrate to Sequoia, the first step is to have new source creation generate a PGP key pair using Sequoia, and then use those keys for encryption/decryption.

This work will include:

• Writing Sequoia bindings for Python
• Converting key storage to be in the database
• Switching over new source creation to use those functions and store said keys in the database.

Most of this work already exists in the oxidize branch

[legoktm]

What is coming in this PR:

• Storing journalist public key out of the GPG keyring - including a hopefully not too hacky postinst migration
• DB schema change, adding 3 columns to the source table
• Changing all the EncryptionManager.encrypt_* functions to use redwood, and sometimes the decrypt function
• Actually switching the new source creation to Sequoia

Honestly I've spent most of the time getting tests to pass, including:

• Getting rid of all EncryptionManager._gpg direct calls and fixing all the other EncryptionManager calls really.
• Encrypting the journalist test key with a passphrase because Sequoia wants that

Except some tests are failing with redwood.RedwoodError: OpenPgp(Missing session key: No session key decrypted) in my new utils.decrypt_as_journalist() function. I'm trying to get everything else passing before I dive into this one.

[legoktm]

The missing session key issue seems to be with encrypting a message for multiple recipients. Here's a pure-Rust test case that fails:

    #[test]
    fn test_multiple_recipients() {
        // Generate 2 keys
        let (public_key1, secret_key1, _fingerprint1) = generate_source_key_pair(
            "correcthorsebatterystaple",
            "foo1@example.org",
        )
        .unwrap();
        let (public_key2, secret_key2, _fingerprint2) = generate_source_key_pair(
            "correcthorsebatterystaple",
            "foo2@example.org",
        )
        .unwrap();

        let tmp = NamedTempFile::new().unwrap();
        // Encrypt a message
        encrypt_message(
            vec![public_key1, public_key2],
            "Rust is great 🦀".to_string(),
            tmp.path().to_path_buf(),
        )
        .unwrap();
        let ciphertext = std::fs::read_to_string(tmp.path()).unwrap();
        // Verify ciphertext looks like an encrypted message
        assert!(ciphertext.starts_with("-----BEGIN PGP MESSAGE-----\n"));
        // Decrypt as key 1
        let plaintext = decrypt(
            ciphertext.clone().into_bytes(),
            secret_key1,
            "correcthorsebatterystaple".to_string(),
        )
        .unwrap();
        // Verify message is what we put in originally
        assert_eq!("Rust is great 🦀", &plaintext);
        // Decrypt as key 2
        let plaintext = decrypt(
            ciphertext.into_bytes(),
            secret_key2,
            "correcthorsebatterystaple".to_string(),
        )
        .unwrap(); // <-- fails here
        // Verify message is what we put in originally
        assert_eq!("Rust is great 🦀", &plaintext);
    }