Add stock_article_selection; 5 small changes

Ignore secret_token.rb
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This should be the default, right? It is the same for these *.yml files which are meant to be created from the sample.

Add stock_article_selection
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This 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). Note that there are two commits. The second one improves some things.

If you have time, please give some feedback.

Fix order.created_by
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In fact obsolete, because you have already done the same.

Fix empty dd in horizontal dl
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This fixes a mean css bug(?) of twitter-bootstrap when some dd nodes are empty in a horizontal definition list like, e.g. https://foodsoft.aboutsource.net/beisswat/suppliers/4 . The fix was done as suggested by a user at https://github.com/twitter/bootstrap/issues/4062 .

Show customer number in fax pdf
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This is a feature request, too. In html view, it is already there. Note that I had to shift the date downwards in order to generate some vertical space.

Prevent stock_articles from appearing in supplier article index
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Actually, it's strange that I havent recognized this bug before. If there are stock_articles for a certain supplier, showing the article index page leads to a crash. This is a fix for that particular crash in the case it is not intended to show stock_articles there at all. I am not sure about that because
* there are two different css styles but
* the latest version in use does not show stock_articles there neither.