Rework multiple file upload

[mshibuya]

There's many quirkiness in current multiple file upload feature. It should come with

• ability to append a file without discarding existing ones
• ability to delete an individual file
• ability to reorder files without re-uploading

in the next release.

[mshibuya]

Request for delete feature:

• Question: Is there no way to remove a singe image with Multiple file uploads? #1704

Request for append feature:

• Files get overwritten #2291
• Skip removing old uploaded images when uploading new images #2288
• ActiveRecord setters and unique filenames with mount_uploaders/multiple #2247
• Add more files and remove single file when using default multiple file uploads feature #2212
• Database record field does not retain old files. #2205
• Update multiple files without replace #1844
• Append more files  #1545

Request for reorder feature:

• Possible to reorder multiple attached files without reuploading? #2337

Other multiple upload related issues:

• Multiple file uploads: Only saves the first image string in the model's field, rather than whole hash/json #2360
• Multiple file upload mobile Safari IOS < 10 #2265
• Remove from multiple files #2257
• Multiple uploaders mounted to one model, full_filename receives wrong filename #2172
• multiple mounts: ApplicationRecord instance "to_json" result is weird. #2106
• Can not delete all images on a upload that handles multiple files #1990
• Uploading multiple images with same image name doesn't work #1855
• access array key of file when using mount_uploaders #1771