Make sign in function

Made the simple sign in functionality, and what that includes:

* Abstract form to partial
* Add dashboard constraint to routes 
* Organize homes#show

Trello card: https://trello.com/c/rXMzCjcs/19-as-a-user-i-should-be-able-to-sign-in
For extra explicitness, you could wrap the route to `homes#show