media assets: add duration column.

Add a column for tracking the duration (length) of videos and
animations. The duration will be null for static images.

db/migrate/20210926123414_add_duration_to_media_assets.rb

@@ -0,0 +1,6 @@
+class AddDurationToMediaAssets < ActiveRecord::Migration[6.1]
+  def change
+    add_column :media_assets, :duration, :float, null: true, default: nil, if_not_exists: true
+    add_index :media_assets, :duration, where: "duration IS NOT NULL"
+  end
+end