Refactor: copy_assets instead of copy_asset

[clarfonthey]

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Are you doing the PR on the next branch?

The site component has a private copy_asset function that just runs copy_file_if_needed, but the copy_asset function itself is part of a larger loop which is repeated in both page and section. This replaces copy_asset with a copy_assets function that contains the larger loop.