Use md5 hash of app public key to identify a user

[friedger]

This PR

• uses the md5 hash of the app public key to identify users instead of usernames.

[CLAassistant]

All committers have signed the CLA.

[hstove]

Overall, something like this is a good idea. Thanks @talhasch for the submission!

• Needs tests
• Won't this break for all existing users? They'll have data stored under their "username" ID

I see the window._reservedUsernames thing, but there doesn't seem to be any place where that variable is set. I would assume it's there to not be a breaking change, but it doesn't look fully working yet.