Part zip file downloads.

[Orion Pobursky]

I've added the ability to download a zip file of all the unofficial parts/subparts/prims for a particular part. This is only for Parts/Shortcuts and only those that actually have subparts. The download will show as a "Download ZIP rollup" button on the part detail page.

[Gerald Lasser]

Thanks for implementing this!

This will ease the download of unoffficial parts for a many users and make parts easier available for stud.io