Now uses jQuery to compute the position of the help box. Combining
.position() and .offset() means that relatively positioned elements
in the help box's ancestry don't affect its final position.

For very small screens, the help box now resizes to fit the screen.
For larger screens it's capped at 500px (rather than using percentage
widths as before) to avoid the width being dependent on parent
elements.

Change-Id: Ib65b9795ac25789d653fd20cbe2b2cd46d2d8c75
Signed-off-by: Jono Mingard <jonom@catalyst.net.nz>