deprecate article unit convertion from ST to KI (german foodcoops legacy)

[yksflip]

in models/article.rb is this convert_units special case that converts articles with the unit 'KI' (Kiste) to 'ST' (Stück). The comment says, that it's some legacy feature for german foodcoops. With the upcoming article_versions, this get's super obsolete. Will anyone still relies on that or can we deprecate it after we merged article_versions?

https://github.com/foodcoops/foodsoft/blob/master/app/models/article.rb#L195
https://github.com/foodcoopsat/foodsoft_hackathon/blob/master/app/models/article.rb#L201