uploads: add upload_media_assets table.

Add a join table that allows multiple media assets (images or videos) to
be attached to uploads. This is for a future ability to upload multiple
files at once.

db/migrate/20220124195900_add_upload_media_assets.rb

@@ -0,0 +1,12 @@
+class AddUploadMediaAssets < ActiveRecord::Migration[7.0]
+  def change
+    create_table :upload_media_assets do |t|
+      t.timestamps null: false
+      t.belongs_to :upload, null: false
+      t.belongs_to :media_asset, null: false
+    end
+
+    add_foreign_key :upload_media_assets, :uploads, deferrable: :deferred
+    add_foreign_key :upload_media_assets, :media_assets, deferrable: :deferred
+  end
+end