Add grabbing files from pantheon to `composer site:import`

[oliviaartz]

As it is this method grabs the database, but it leaves out all the image assets (and whatnot) referenced by the site.

[CybersecDan]

I feel like composer site:import shouldn't sync files as well, as the files aren't always necessary for local development. I'd be interested to hear what others think though.

[rbayliss]

I agree. Ideally, we'd want to avoid grabbing files altogether by using something like stage file proxy.