Encrypt messages with team key

[adrastaea]

Description

As a first step before having tiered message encryption, we should encrypt every message with the team key. In storage.service:sendMessage, encrypt the message before adding it to the channel db.

Acceptance Criteria

1. Encryption mechanism / libraries pass review by e.g. Alishah or some other expert
2. Message model includes key name and generation
3. Messages get encrypted before being added to channel db
4. (maybe) Differentiate public and private channels, keep the publicChannelsRepo infrastructure as a way to send unencrypted messages, and add a privateChannelsRepo which gets encryption Just use the team key
5. (maybe, up to implementer) Pull channels functions out of StorageService, create a Channels module, and move all the message sending, receiving, encrypting, and decrypting logic there

The key question on 4 is: can we do this in a very quick and minimal way that is also clean and likely to not introduce bugs? If so, we should do that. If not, we should do as much as necessary to make sure it's going to be clean and bug free.

[holmesworcester]

Note: have a look at how background notifications are working on Android before tackling this.