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.

script/fixes/109_generate_media_asset_file_keys.rb

@@ -0,0 +1,8 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+MediaAsset.where(file_key: nil).parallel_each do |asset|
+  asset.update_columns(file_key: MediaAsset.generate_file_key)
+  puts "id=#{asset.id} file_key=#{asset.file_key}"
+end