Fixes #90 - steward carousel

- Makes the Steward and Trail next/prev buttons act like a carousel, so
that when they reach the end or beginning they loop around to the
beginning or end.
- Also hides the chevron arrows for the trails when there is only 1 of
1 arrows.
Merge pull request #125 from codeforamerica/90-carousel

Fixes #90 - steward carousel