implications: refactor calculation of implied tags.

Refactor to use a recursive CTE to calculate implied tags in SQL, rather
than storing them in a descendant_names field. This avoids the
complexity of keeping the stored field up to date. It's also more
flexible, since it allows us to find both descendant tags (tags that
imply a given tag) as well as ancestor tags (tags that are implied by a
given tag).

db/structure.sql

@@ -2883,7 +2883,6 @@ CREATE TABLE public.tag_implications (
     id integer NOT NULL,
     antecedent_name character varying NOT NULL,
     consequent_name character varying NOT NULL,
-    descendant_names text[] DEFAULT '{}'::text[] NOT NULL,
     creator_id integer NOT NULL,
     forum_topic_id integer,
     status text DEFAULT 'pending'::text NOT NULL,
@@ -7352,6 +7351,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200119184442'),
 ('20200119193110'),
 ('20200123184743'),
-('20200217044719');
+('20200217044719'),
+('20200223042415');