This introduces a new djblets.secrets module, which will be the home

of any future crypto/secrets-related work going forward.
This currently contains djblets.secrets.crypto, a collection of AES

encryption/decryption utilities that wrap the cryptography module.

This is a near-straight port of reviewboard.scmtools.crypto_utils,

with the exception that encrypt_password() has been renamed to

aes_encrypt_base64() and decrypt_password() has been renamed to

aes_decrypt_base64(). This is to help make it clear what form of

encryption and results are expected, and to allow room for other similar

utilities in the future.
cryptography is now a hard dependency of Djblets, rather than a soft

dependency, as Djblets-shipped code will be making use of this module in

the future.

Unit tests pass.