Implement order details

[fabi321]

This adds the possibility to add information about current orders to items.

I'm not sure, how to properly handle translations, and sqlite backwards migrations.

Fixes #217

[jbtronics]

I do not thing it is really useful to track orders with a simple number at part level, the old Part-DB versions did something similar (with the still existing order_quantity field), but it was quite limited and not so useful for many applications.

I think a order-centered system would be more flexibile and useful. Users can create Orders (as a new entity), assign parts and amounts to it and track its status via it (maybe you could even allow to order directly from Part-DB via the distributors APIs). That way you can have multiple orders of the same part in parallel (without loosing detailed track), can store metadata (like who ordered, and when) and when the order arrives you can easily book in all parts of this order, without needing to do that for each part at once.

This also allows to look for historic orders and other things, which should be useful, and which would not be possible by simply storing the currently in order state amount of parts.