Remove added files since last snapshot

Demo should be capable to remove any new files since the last snapshot. It should only deal with files in the configured upload path of the site, so an admin can add other files (f.e. modules or themes) in other paths.

This could be done in two ways:

1. Just use the date of the last reset to find all files newer than that date.
2. Create a separate file storage where all files are copied during a snapshot. This would take into account that a file could be edited (not added) since the last snapshot. On reset, all files that are not in the snapshot are deleted and all altered files would be overwritten with the version in the snapshot file storage.