api,db: add comments count endpoint

Closes https://gitlab.com/commento/commento-ce/issues/27

db/20180923002745-comment-count.sql

@@ -0,0 +1,15 @@
+ALTER TABLE pages
+  ADD commentCount INTEGER NOT NULL DEFAULT 0;
+
+CREATE OR REPLACE FUNCTION commentsInsertTriggerFunction() RETURNS TRIGGER AS $trigger$
+BEGIN
+  UPDATE pages
+  SET commentCount = commentCount + 1
+  WHERE domain = new.domain AND path = new.path;
+
+  RETURN NEW;
+END;
+$trigger$ LANGUAGE plpgsql;
+
+CREATE TRIGGER commentsInsertTrigger AFTER INSERT ON comments
+FOR EACH ROW EXECUTE PROCEDURE commentsInsertTriggerFunction();