Replace dots in review app subdomains

While dots are valid, they don't work with our SSL certificate. I've already
adapted the hats CI workflow to expect this change.

app/models/review_app.rb

@@ -73,8 +73,8 @@ def computed_environment
     ]
   end
 
-  # replace underscores and downcase - not valid as subdomain
+  # replace dots & underscores and downcase - not valid as subdomain
   def sanitized_subject
-    subject.downcase.gsub('_', '-')
+    subject.downcase.gsub(/[._]/, '-')
   end
 end