Integrate !(de)crypt with !notes better

[legoktm]

The Earwig Mini-Wiki is global, so messages that need to stay private for a channel need to be encrypted.

I'm imagining that each channel can set an encryption key that the bot stores. Then a wiki administrator can enable encryption for a page:

<Earwig> !channelkey set foobar
<EarwigBot> Key set for channel ##this-is-a-test
<Earwig> !notes encrypt Supersecret
<EarwigBot> Content for "Supersecret" is now encrypted.

At this point, anyone in the ##this-is-a-test channel can seamlessly read and write to the "Supersecret" page, but in the backend the bot is encrypting and decrypting it. If someone in a different channel tries to read or write, they'll use the wrong encryption key and it'll appear as gibberish.