Support user authentication in mobile app

This pull request includes all the changes needed to support user authentication in germ-tracker mobile app.  The changes include adding new signin, signup and signout states, as well as updating and adding Auth related services. 

In the server-side, we now have new signin and signup handlers for /api/users/signin and /api/users/signup routes which create a new user if not exist and return a JWT token as a response.

Please note that there are a few areas that are not fully implemented yet such as hashing a password, error handling.  I will create follow-up tasks to address those issues.

cc: @jjmerino, @kmeurer, @jamesongamble, @suprbh