Implemented new Achievement and Badge: Invited a Friend (Fixes #8615) (#8819)

* Added text to locale

* Added achievement to content and libs

* Added achievement modal

* Added achievement to notification model and controller

* Added achievement to user schema

* Grant achievement to inviter when user registers using emailed link

* Fix icon name

* Added integration test

* Fix linting

* Added sprite

website/common/script/content/achievements.js

@@ -112,6 +112,11 @@ let basicAchievs = {
     titleKey: 'joinedChallenge',
     textKey: 'joinedChallengeText',
   },
+  invitedFriend: {
+    icon: 'achievement-friends',
+    titleKey: 'invitedFriend',
+    textKey: 'invitedFriendText',
+  },
 };
 Object.assign(achievementsData, basicAchievs);