Use buttons for switching to/from directions.

Previously, used a small text link which wasn't very usable. Now
it uses larger buttons which are (hopefully) more usable. Also
this sorts out the weird right-hanging text boxes for the to/from
in the directions sidebar - now they look like proper text input
boxes.

However, the "back to search" button and the directions submit
buttons are weirdly large and I don't quite understand why.
Hopefully an easy fix for someone who knows the arcana of CSS
better than I.