artists: rename is_active flag to is_deleted.

Rename is_active to is_deleted. This is for better consistency with
other models, and to reduce confusion over what "active" means for
artists. Sometimes users think active is for whether the artist is
actively producing work.

db/structure.sql

@@ -594,7 +594,7 @@ ALTER SEQUENCE public.artist_versions_id_seq OWNED BY public.artist_versions.id;
 CREATE TABLE public.artists (
     id integer NOT NULL,
     name character varying NOT NULL,
-    is_active boolean DEFAULT true NOT NULL,
+    is_deleted boolean DEFAULT false NOT NULL,
     is_banned boolean DEFAULT false NOT NULL,
     other_names text[] DEFAULT '{}'::text[] NOT NULL,
     group_name character varying DEFAULT ''::character varying NOT NULL,
@@ -7307,6 +7307,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200123184743'),
 ('20200217044719'),
 ('20200223042415'),
-('20200223234015');
+('20200223234015'),
+('20200306202253');