[Frontend] Login Page Design&Implementation

[berkaydoner]

We have to implement a login page where users can sign in to the app using their username/email and password.
I think a simple design with icon/app name above, two form elements at the middle and submit button at the bottom should be good enough.
Also, this login page should interact with login endpoint of backend and it should create a session for the user.

[berkaydoner]

I have implemented the login page with two text fields. There are also links to the signup and forgot password pages. #183

[berkaydoner]

I connected the login functionality to the backend and added a error message for the case where the response indicates an error. If sign in is successful, user is navigated to the home page.

[berkaydoner]

I merged the functionality. I could not added the session functionality as the backend part was implemented a little bit late, but I will add it as soon as possible in a separate branch.