production: Add relates from product to BOM input and BOM output
It is quite useful to have two additional entries in the relate form for
product to answer the following questions with ease:
* Where is a particular product used as BOM input?
* Where is a particular product used as BOM output?
We have at least two use cases for such a change:
* Simplifies editing of BOMs in general by navigating directly from the
product to the BOM in question.
* In order to disable/replace a product in a BOM, one needs to find all
occurrences.
If this is of interest, we can submit our patch.