DEV: Use consistent interpolation key format in translations

From now on client strings can easily be reused on the server and you don’t have to think about choosing the right format anymore.

config/locales/server.en.yml

@@ -899,7 +899,7 @@ en:
       short_description: 'A violation of <a href="%{base_path}/guidelines">our community guidelines</a>'
       long_form: "flagged this as inappropriate"
     notify_user:
-      title: "Send @{{username}} a message"
+      title: "Send @%{username} a message"
       description: "I want to talk to this person directly and personally about their post."
       short_description: "I want to talk to this person directly and personally about their post."
       long_form: "messaged user"
@@ -4826,9 +4826,9 @@ en:
     title: "Reminder about old credentials"
     body: |
       Hello! This is a routine yearly security reminder from your Discourse instance.
-      
+
       As a courtesy, we wanted to let you know that the following credentials used on your Discourse instance have not been updated in more than two years:
-      
+
       %{keys}
-      
+
       No action is required at this time, however, it is considered good security practice to cycle all your important credentials every few years.