Add stock_article_selection for deleting multiple articles at once

It is a feature request of my foodcoop: Many existing unused stock articles make it hard to get a nice overview. Deleting those unused stock articles is a time consuming job - you have to
1. unhide them
2. search for one in the long list
3. click 'delete'
4. confirm and wait for page load (a couple of seconds) .

This is an attempt to make it easier: After unhiding, you can select multiple unused articles via checkboxes and then create a ```stock_article_selection```. Those articles in a selection can be deleted at once. For the reason for this (complicated?) way instead of direct deletion by checkboxes see https://github.com/balkansalat/foodsoft/issues/21 (I tried to keep it RESTful).

If you have time, please give some feedback.