feat(app): support more login methods

[nicholaschiang]

Right now, we only support a Google login via redirect. It'd be nice to be able to collect all the information that they give us (in a different way) and add support for other sign-in methods. We'll need to collect:

• Display name (e.g. Nicholas Chiang)
• Email address
• Photo URL

There's some other information that Google gives us that we do not use. Priority 1 is to add email login/signup support. Then we can deal with supporting Facebook, Instagram, GitHub, and other OAuth providers.

Also note that we're using the Firebase Auth SDK to login users.

[jajooshraddha]

Firebase SDK provides methods for password authentication so the student need not compulsorily have a google account. It also has other signin methods such as Facebook, Github etc.