Bug 19166: (follow-up) Reformat the new adjustments markup

Bug 19166: (follow-up) Reformat the new adjustments markup

This follow-up makes some changes to the way invoice adjustments are
displayed and added:

 - Move this section into a <fieldset> to set it off visually from the
   other sections of the page.
 - Take the "add adjustment" fields out of the table and display them in
   a standard way.
 - Hide the "add adjustment" fields by default, shown via "add an
   adjustment" link.
 - Add deletion confirmation to adjustment deletions.

To test, add, delete, and change invoice adjustments as instructed in
the original test plan.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>