Create rake task to delete orphaned imports

[maxkadel]

Create a rake task to be run after the one created in #1204 in order to delete the import and export files or directories that have been successfully copied.

Acceptance criteria

• Accept arguments for new file back end and old file back end
• Accept argument to target either imports or exports
• Checks to ensure that the file has been successfully copied to the new back end prior to making any changes
• Deletes old file
• Logs deletion

Implementation notes

• Should probably kick off a background job
• May want to maintain a separate move / deletion log?