Admin can now write posts

Posts can be displayed on the website. Subscribers are automatically
notified by email. This gives the opportunity to track explicitly how
many people followed the link provided in the emails sent without being
intrusive (no invisible image).

tests/api/main.rs

@@ -4,5 +4,6 @@ mod health_check;
 mod helpers;
 mod login;
 mod newsletters;
+mod posts;
 mod subscriptions;
 mod subscriptions_confirm;