Feature: Log In - Githubissues

[x] On every page of the site, a "Log in" button (or drop down menu containing "Log in") must be at the top-right of the page.
[x] If using a drop down menu, the menu must contain a "Log in" menu option.
[x] If using a drop down menu, the menu must contain a "Sign up" menu option.
[x] Upon clicking the "Log in" button or menu option, it opens a modal pop-up that prompts the Username and Password input boxes and a "Log in" button.
[x] Within the modal, the "Log in" button must be disabled anytime the username is less than 4 characters or the password is less than 6 characters.
[x] Attempting to log in with an invalid username or password must prompt the error message "The provided credentials were invalid".
[x] Upon logging in with a valid username and password, it must successfully log-in the user and sets their session cookie.
[x] Upon logging in with a valid username and password, the "Log in" and "Sign up" buttons at the top of the page are hidden.
[x] Upon logging in with a valid username and password, it shows the User Menu Button (see next feature).
[x] In the log-in modal, an extra link or button is available with the label "Log in as Demo User". Upon clicking this "Log in as Demo User" button, it will log the user into the "demo" account.
[x] Upon closing the log-in modal, it resets errors and clears all data entered. When it reopens it will be in the default state (empty inputs and disabled button).

gracecizma / HeirBnB