Master quantity units: make default conversions editable

[dominiczedler]

And display them as "read-only" on the master product page.

[pasnox]

I would really want this feature implemented. I started using grocy and the application led me to a wrong usage, I had to add conversion units for each product added while in fact I would have wanted default conversion. Also, this was added for a version milestone you already reached by far, is this really made or just outdated?