Google login

This itself isn't too complicated. We just use the client side google auth package and send the resulting token or whatever to the server where we issue our own token etc. However, we need an intelligent system for merging accounts.

I think it should be quite explicit when you're first signing up vs logging in, i.e. don't allow people to log in with google if they haven't signed up already. Or do but prompt them first. The reason for this is because we want to allow people to merge their accounts, but if they make an account with username/password and then login in with google they will have two accounts, and won't be able to merge them after that point.

So:
* log in with google on login screen
* register with google on register screen
* if you log in with google but don't have an account yet, it asks you if you want to create one, or if you want to merge with an existing account - perhaps at this point we can pass on the social auth data and automatically merge them after a successful login
* you can log in and then in account settings "connect other login methods"