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/migrate/20200223042415_drop_descendant_names_from_tag_implications.rb

@@ -0,0 +1,5 @@
+class DropDescendantNamesFromTagImplications < ActiveRecord::Migration[6.0]
+  def change
+    remove_column :tag_implications, :descendant_names, "text[]", default: "{}", null: false
+  end
+end