UI Responsiveness

The main gain with this is all pages should now look solid on small screens(you can easily test this by resizing your [desktop] browser, too).

- There's now a respond-to mixin that takes an argument of a screen 'size' keyword(e.g., small-screen).
- I think the current's designs weakpoint of explicit dimensions for the app-listing element and the complication of possibly long(ish) description text is going to have to be addressed at some point, but not today.

My judgments are passed in shadow.