`file_key` is a random 9-character base-62 string that will be used as
the image filename in the future.
`is_public` is whether the image can be viewed without authentication or not.
Users running downstream boorus must run `bin/rails db:migrate` and
`script/fixes/109_generate_media_asset_file_keys.rb` after this commit.
A MediaAsset represents an image or video file uploaded to Danbooru. It
stores the metadata associated with the image or video. This is to work
on decoupling files from posts so that images can be uploaded separately
from posts.
