api, frontend, db: add comment sorting

Closes https://gitlab.com/commento/commento/issues/215

db/20191204173000-sort-method.sql

@@ -0,0 +1,10 @@
+-- Default sort policy for each domain
+
+CREATE TYPE sortPolicy AS ENUM (
+  'score-desc',
+  'creationdate-desc',
+  'creationdate-asc'
+);
+
+ALTER TABLE domains
+  ADD defaultSortPolicy sortPolicy NOT NULL DEFAULT 'score-desc';