Create user object in database on signin

[AryenSinghal]

If a user signs in for the first time, a new user object should be created and saved to the database.

• Generate a new user_id. This can just be an increasing counter, as it's just an identifier, not a security feature.
• Populate the necessary fields automatically (name, email, etc.). Rest should be left blank.
• Update timestamps as needed.

If an existing user signs in, don't create new object.

• Verify that the access tokens and auth stuff is still valid. If not, generate new ones.
• Update the lastLogin timestamp.