Clarification of minimum order quantity / default order note for a supplier

[twothreenine]

See discussion in #838

Problem: min_order_quantity was originally intended to be just a textual description and is therefore a string. This is confusing to users, some don't know whether to enter 25 Euro, 25 €, or just 25, or they enter something like 20 kg.
Later, a very useful feature to only auto-send the order to the supplier if the minimum order quantity is fulfilled was added, which compares this string to the order sum (I believe it is the gross sum, but this should be clarified in a tooltip as well).

My suggestion:

1. Add a text field supplier.order_note which prefills the order.note field of each order opened for this supplier. This could be used for various information like minimum order quantity in crates etc., transport cost, link to supplier's website etc. (Additionally, we could also extend the custom fields feature to orders, but that would take more work.)
2. Then we could migrate min_order_quantity to a monetary field (rename it to min_order_value?), detect all non-numeric / non-monetary data saved in it, and move it into supplier.order_note (with a label Minimum order quantity: in the respective locale). E.g. 25 Euro or 25 € should be converted to 25, but 20 kg should be moved to supplier.order_note.
3. Add currency symbol and clarification tooltip to minimum order value.