Parallel validator and migrations

[lopho]

• Use threads to parallalize validation and migration.
• Small cosmetic changes to warning messages in validation.
• Skip unneeded operations in migrations
  • don't resize if size is correct
  • don't convert to RGB if it already is
  • copy file instead of saving with pillow if it's unchanged
• Keep image format of input images (webp->webp, png->png ...)
  • old impl converted to jpeg at 75 quality -> artifacts
  • now quality 95 for jpeg and 100 for lossless webp (e.g. compression level)
• fixes #52