Auto-guess expiration dates

[lentschi]

(Originally brought up by @sap7772000zilla via email and @benfreu via Slack)

When an article is entered, that has been added before (by any user in the db), the server should try to deduce the expiration date of the new entry (by adding the average expiration duration to the current date). The estimated date should then be sent to the client and serve as the default expiration date in the new-entry-form.

Another way to solve this would be through categorization (s. #3). For example a yoghurt won't last as long as a closed can of beans. However, since categorization can be quite arbitrary (again s. #3), I would suggest using deduction on base of the article (identified by the barcode) only.