Handle photos with multiple files better

[derneuere]

Describe the enhancement you'd like
Right now, we just save all different files for a photo in a string. However, this limits a lot of options. We do not what type of files these are and what the hash is etc. This makes it impossible to handle things like live photos, raw photos with thumbnails, movies with thumbnails, stacking similar images or create version that were edited in a future photo editor.

Steps to do:

• Create a file model
• Add a files field to photo
• Add a main file field to photo
• Migrate current image_paths to files
• Fix about a million lines that used image_paths
• Make sure that every action still works
• Migrate image_hash to file
• Use UUID or ULID as a primary key for photo model instead
• Add a semantic_image_hash field

Describe why this will benefit the LibrePhotos
This is needed to implement multiple open issues like #287 #481