Others: UI does not support smaller screens

[crazoter]

⚠️ Issue:
UI is not functional when the page is zoomed in using the default Windows setting for some computers. For example, login Page is not properly shown at the default Windows scale (Chrome 100% zoom):

Windows settings:

⬇️ Reproduce:
Zoom in using your browser.

✅ Expected behaviour:
At the very least, the Sign up button should be visible. Consider:

1. Using flexbox CSS to vertical align the content. https://css-tricks.com/snippets/css/a-guide-to-flexbox/
2. Allowing it to be scrollable on overflow-y