[FEATURE] JS changes available BE variant stock

[rintisch]

With activated stock handling for BE variants the JavaScript which changes the prices does also change change the available stock.

Requirements:

• This requires that the select view contains a data atrribute 'data-available-stock' with the stock for the BE variant.
• Furthermore some HTML has to be present which will be used to insert the available stock.

Both changes will be made in a commit in EXT:cart_products.

!!! This commit changes also the buggy behaviour that reselecting "Please choose..." from the select field shows the price of the last chosen item. Instead a   will be inserted (to display no price while avoiding 'jumping' layout.

Fixes: #487