Bug 27289: Template tweaks for point of sale page

Bug 27289: Template tweaks for point of sale page

This patch makes a couple of minor changes to the point of sale page in
order to make it consistent with other pages in the staff interface:

- Buttons inside table cells should have the "btn-xs" class.
- Required fields should have a "required" class and should have a
  "Required" label.

Also changed: Replaced <input type="number"> according to coding
guidelines.

To test, apply the patch and enable the EnablePointOfSale and
UseCashRegisters system preferences if necessary.

- On the Point of Sale page, add some items to purchase. Confirm that
  the "Add" buttons in the "Items for purchase" table and the "Remove"
  buttons in the "This sale" table are correctly sized.

- In the "Collect payment" form, confirm that "Amount tendered" and
  "Cash register" fields are styled red with "Required" text after them.
- Confirm that the "Amount tendered" field will only accept numeric
  input.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>