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/structure.sql

@@ -3779,6 +3779,13 @@ CREATE INDEX index_media_assets_on_image_width ON public.media_assets USING btre
 CREATE INDEX index_media_assets_on_md5 ON public.media_assets USING btree (md5);
 
 
+--
+-- Name: index_media_assets_on_md5_and_status; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE UNIQUE INDEX index_media_assets_on_md5_and_status ON public.media_assets USING btree (md5) WHERE (status = ANY (ARRAY[100, 200]));
+
+
 --
 -- Name: index_media_assets_on_status; Type: INDEX; Schema: public; Owner: -
 --
@@ -5049,6 +5056,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20211014063943'),
 ('20211015223510'),
 ('20211018045429'),
-('20211018062916');
+('20211018062916'),
+('20211023225730');