Bring in Fee Sheet improvements from recent IPPF work.

Bring in Fee Sheet improvements from recent IPPF work.

This refactors much of the Fee Sheet into a new pair of classes: FeeSheet to contain
the "business logic" and FeeSheetHtml which extends it and adds functions to facilitate
generation of HTML.  Other improvements include:

o Line item price levels. A new column at the line item level that appears if you
  have more than one price level to choose from.
o Button to Add More Items, which is a step towards supporting multiple checkouts
  which will be in a following commit.
o Button to re-open the encounter.
o Improved logging.
o Some fixes related to translation.
o New feature with checkboxes instead of drop-lists for adding new items.
o Corresponding updates to drugs.inc.php and billing.inc that will be used later.
o Fixed failure of the review feature due to knockout library being moved.