security: Updating AES mode to one that does not require a unique IV

[ZephSibley]

Looking at the config around the IV and the use of it in the code it seems that the system would encrypt more than one message with the same IV, making the messages vulnerable to attacks explained here

This PR changes the mode to CBC, which can use a fixed IV, and has minimal drawbacks. An alternative to this would be to use a nonce as the IV and send it along with the ciphertext for use in decryption.

Thanks!

[dskvr]

Also see https://github.com/rgrizzell/Armachat-circuitpython/blob/refactor/docs/guides/user_guide.md, there was some work done on a refactor but unclear where it stands at this moment

[bobricius]

There is many non working thinks. etc addressing and communication, I was not able run this refactor fork