Fix to previous answers css

- Previous answers block width now follows width of current questions above.
- With long previous questions the block no longer expands to far to the right.

The local static media-down mixin is the local conditionals2 which static uses. It is only here for a short time until the forking of the smart answers css is completed.

https://www.agileplannerapp.com/boards/105200/cards/5834
Merge pull request #1148 from alphagov/fix-previous-answers-table-width

Fix to previous answers css