make a landing page

observed: there's no landing page, but directly a login form
expected: there should be a landing page, and a login UI both at the superior right corner, or through a button somewhere in the landing page, in a standard way:

<img width="1094" alt="Screenshot 2020-12-22 at 14 36 18" src="https://user-images.githubusercontent.com/2310732/102894296-2b124200-4463-11eb-98dc-78b17770d665.png">
