Remove hardcoded passcode for encryption

[kautukkundan]

I thought for a first PR it would be good to just get the encryption working. I figure as a separate PR I'll store the pass key in memory.

If anyone has any recommendations on how I should do that, let me know. I know that metamask uses obs-store to keep the password in memory.

Originally posted by @blakecduncan in #286 (comment)

[jacque006]

I think as long as you clearly label it as a password, and we have some window where it will be cleared due to inactivity or just after a certain amount of time you can use almost anything. Might make sense as a cell.

[blakecduncan]

I think as long as you clearly label it as a password, and we have some window where it will be cleared due to inactivity or just after a certain amount of time you can use almost anything. Might make sense as a cell.

@jacque006 I've been thinking I might have to do that. The original plan was to keep the CryptoKey in memory for encoding and decoding the data. But I've been stuck on that because if I keep the key in memory in the background script then I don't think it's easy for the extension dapp to access the key. And we read/write to local storage from both the dapp and the background script.

Update: I just realised we have memory cells and local cells. It would be great if I could use a memory cell but I'm not sure how I can easily pass it back and forth between the background script and the dapp. Which is making me think I would need a local cell.