Make sure different auth providers don't override each other and link them together instead

[sepsol]

Currently, if a user who has signed up with an email (especially Gmail) and then, later on, decides to sign up with Google, it'll actually override his / her previous auth information on firebase and they'll not be able to use their email and password to log in anymore.

This can be fixed by linking different providers together. We may need additional screens or modals to implement this.