FIX: New notification type needs some i18n string (#304)

config/locales/client.en.yml

@@ -127,3 +127,6 @@ en:
               label: Users in working hours
     notification_reason:
       user: "You will see a count of new replies because this topic was assigned to you."
+    notifications:
+      popup:
+        assigned: "%{username} assigned you"

config/locales/server.en.yml

@@ -74,3 +74,6 @@ en:
       random_assign:
         title: Random assign
         no_one: "Attempted randomly assign a member of @%{group}, but no one was available."
+  discourse_push_notifications:
+    popup:
+      assigned: "@%{username} assigned you"