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/server/models/user/schema.js

@@ -117,6 +117,7 @@ let schema = new Schema({
     royallyLoyal: Boolean,
     joinedGuild: Boolean,
     joinedChallenge: Boolean,
+    invitedFriend: Boolean,
   },
 
   backer: {