Create a POST route so a user can sign in

[chrislaupama]

We may need a POST route to allow a user to sign in. This may send back to the client information about the user.

Tasks:

• [ ] Refer to the wiki to see how the routes may need to be setup.
• [ ] Create route.
• [ ] Create db function.
• [ ] Write tests for this route.
• [ ] Write tests for db function.