Bug 5186 - allow tax rates to be set to zero (master)

Bug 5186 - allow tax rates to be set to zero (master)

This allows the tax rate for a vendor to be set to zero. Previously, a
zero meant that the system default was used. Now, zero means no tax, and
to have it be the default the field should simply be left empty.
Additionally:
* this will now show on the vendor display if the tax value is from the
  system default
* this includes a database update that changes all the existing 0.00 tax
  settings to be NULL, which preserves existing behaviour.
* this now saves the tax value supplied for new vendors

Note: this patch applies against master

Signed-off-by: Nicole Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>