Resize signin/signup popup

[ddfridley]

The signup popup currently limits it's height to 90vh and adds a scroll bar if the popup is too big for the window.
Instead, we should make the whole component based on 'em' rather than rem, and adjust the font size, if the component renders too large for the viewport.

app/components/signin-signup.js

See this for an example of fitting the fonts-size.

For history on this, see also: #238

• add stories to the existing file that test this feature.