Add filenames to registry via trigger/hook/event

[dstufft]

Instead of adding filenames to the filename registry inline in the upload view, we should be doing this via some sort of hook or trigger. This is probably a good candidate for just using a database level trigger for.

[miketheman]

Relevant code in upload view: https://github.com/pypi/warehouse/blob/ddd1b3507fdf109e7c1339ac64c4c142566f8c71/warehouse/forklift/legacy.py#L1379-L1382

[Delta17920]

@dstufft @miketheman