Remove duplicate import in index.scss

Sass treats 'conditionals' and '_conditionals' as the same file,
the one from the frontend toolkit. Remove the unnecessary second
import.
Refactor out 960px as the page width for GOV.UK

Remove a magic number.
Enclose footer text width in desktop media query

Only shorten the width of the text on desktop, where we want to ensure
that the lines are not too long. On mobile, fill the screen.

Switch to using a Sass calculation with the new page width variable
rather than an arbitrary percentage based on a comment.
Separate out footer-text and description styles

These don't share much in common, so split them out rather
than using a selector with a comma.

Switch them around because that's the order they appear on the page.
Merge pull request #471 from alphagov/sass-high-volume-notes-small-screen

Improve small screen styles for high volume dashboard footers