Don't constraint width for .sortables

Summary:
On iOS, the font size is larger and thus the contents of the .sortables occasionally
spill over. I changed the CSS property from `width` to `min-width` to allow them to expand
if necessary.

Test Plan:
- Run mobile app.
- Check out "Ordering decimals".
- Verify that the contents do not spill outside the sortable cards.
- Do the same for web.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D11829