DEV: Enable unless cops

We discussed the use of `unless` internally and decided to enforce
available rules from rubocop to restrict its most problematic uses.

config/site_settings.yml

@@ -1027,6 +1027,7 @@ posting:
   notify_about_queued_posts_after:
     type: float
     default: 24
+    min: 0
   auto_close_messages_post_count:
     default: 500
   auto_close_topics_post_count:
@@ -1955,6 +1956,7 @@ rate_limits:
   max_prints_per_hour_per_user:
     default: 5
     client: true
+    min: 0
   max_logins_per_ip_per_hour:
     min: 1
     default: 30