Rake task for migrating import and export back end

[maxkadel]

Create a rake task to migrate from one back end for storing imports and exports to another.

Acceptance criteria

• Allow back end migration configuration and rake task run to be separate for imports and exports
• Configurable "from"
• Configurable "to"
• Copies files to new location, leaving old files to be deleted in separate rake task
• Updates any pointers to files in database to new location
• Creates a file in the old directory to indicate that the file or directory has been successfully copied - something like filename.done
• Imports and exports can still be viewed in the UI without erroring