Add a right margin of 1.5em on touch tables.

It's easier for the user to scroll if there's whitespace on both
sides.
Fix bug on tables for touch devices.

We should check for Modernizr touch support client-side,
not on the server :( Update tests accordingly.

I've tested this in Chrome's browser emulator and it appears
to work.
Merge pull request #615 from alphagov/bugfix/make-tables-scrollable-part2

Fix bug in scrollable tables