Reduced SQL load for 'Require Stock to Complete Build' widget on homepage

[eeintech]

Follow-up of #984 (2 SQL queries vs N+1 for build related items)

[SchrodingersGat]

Have you profiled this using the debug toolbar?

[eeintech]

Oh yeah I should have mentioned this!

I have 21 parts in my database. I tested with following URL: /api/part/?stock_to_build=true

With the original code, I had 20 similar queries regarding build items fetching.
With this code, those 20 similar queries are gone and the number of SQL queries went down by 19.

However, the potentially redundant stock item queries are not eliminated. It's a bit more complex as the prefetch_related is embedded in the Part.can_build property method.
But the subset of parts is greatly reduce by the time it reaches this step, so I don't see any major hiccup here.

[SchrodingersGat]

Nice, thanks for implementing this and testing it :)