Allow to synchronise all articles + category matching

The current shared database functionality is geared towards wholesale retailers. When subscribing to a farmer's article list, foodcoops need to import new articles every time. This change allows one to specify how to do synchronisation for a supplier: (1) Import all articles, (2) Import all articles but make them not available yet, and (3) Manually import articles (current functionality).

Also categories from shared articles are copied when there is a matching category, either in name or in description (which is renamed to "import names" in the user-interface - it wasn't really used).

One step closer to automatic synchronisation (see also foodcoops#187).