media assets: add file_key, is_public columns.

`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.

app/policies/media_asset_policy.rb

@@ -13,7 +13,7 @@ class MediaAssetPolicy < ApplicationPolicy
     if can_see_image?
       super
     else
-      super.excluding(:md5)
+      super.excluding(:md5, :file_key)
     end
   end
 end