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

@@ -44,6 +44,7 @@ gem 'puma'
 gem 'scenic'
 gem 'ipaddress'
 gem 'http'
+gem 'activerecord-hierarchical_query'
 
 # needed for looser jpeg header compat
 gem 'ruby-imagespec', :require => "image_spec", :git => "https://github.com/r888888888/ruby-imagespec.git", :branch => "exif-fixes"