media assets: prevent duplicate media assets.

Add a md5 uniqueness constraint on media assets to prevent duplicate
assets from being created. This way we can guarantee that there is one
active media asset per uploaded file.

Also make it so that if two people are uploading the same file at the
same time, the file is processed only once.

db/migrate/20211023225730_add_unique_md5_constraint_on_media_assets.rb

@@ -0,0 +1,5 @@
+class AddUniqueMd5ConstraintOnMediaAssets < ActiveRecord::Migration[6.1]
+  def change
+    add_index :media_assets, :md5, name: "index_media_assets_on_md5_and_status", unique: true, where: "status IN (100, 200)"
+  end
+end