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).

Gemfile.lock

@@ -63,6 +63,9 @@ GEM
     activerecord (6.0.2.1)
       activemodel (= 6.0.2.1)
       activesupport (= 6.0.2.1)
+    activerecord-hierarchical_query (1.2.3)
+      activerecord (>= 5.0, < 6.1)
+      pg (>= 0.21, < 1.3)
     activestorage (6.0.2.1)
       actionpack (= 6.0.2.1)
       activejob (= 6.0.2.1)
@@ -402,6 +405,7 @@ PLATFORMS
 
 DEPENDENCIES
   activemodel-serializers-xml
+  activerecord-hierarchical_query
   addressable
   awesome_print
   aws-sdk-sqs (~> 1)
@@ -473,4 +477,4 @@ DEPENDENCIES
   whenever
 
 BUNDLED WITH
-   2.0.2
+   2.1.2