Migrate file_managed table to file entities

Problem/Motivation

Managed Drupal 7 files need to be migrated to Drupal 8 file entities.

Proposed Resolution

D7's unified file management API (and the fact that files are already entities!) makes this a lot easier than migrating files from Drupal 6 :) We need a simple migration (d7_file) that simply transfers all file_managed rows to Drupal 8 file entities, and copies the actual files to the corresponding Drupal 8 path.

Remaining Tasks

1. Write the migration
2. Write tests
3. Review
4. Commit