Fix visualization scaling and button alignment for RTL.
Always float step button right. Make Run button invisible (rather than
display: none) on step-only levels to preserve layout of other
buttons. Clean up some duplicate CSS.
Reduce button padding on small screens to avoid ugly wrapping.
Move share button to its own line in bounce levels, so that it doesn't
move other buttons around when it appears.
Merge pull request #805 from code-dot-org/responsive-rtl

Make responsive layout work for RTL. Fix button alignment and shrink buttons to avoid wrapping on small screens.