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:

warehouse/warehouse/forklift/legacy.py

Lines 1379 to 1382 in ddd1b35

	# TODO: This should be handled by some sort of database trigger or a
	# SQLAlchemy hook or the like instead of doing it inline in this
	# view.
	request.db.add(Filename(filename=filename))