Guild A/B test and Achievement (#8740)

* WIP(guilds): AB test pester modal

* WIP(AB-test): guild pester cont'd

* fix(style): linting error

* fix(AB-test): markModified and notif enum

* fix(tests): update AB expectations

* fix(modal): remove extra includes

* feat(achievements): add Joined Guild cheevo
Also removes unused achievement sprites, and properly saves counter used in A/B testing

* fix(style): linting error from conflict

website/server/models/userNotification.js

@@ -18,6 +18,8 @@ const NOTIFICATION_TYPES = [
   'GROUP_INVITE_ACCEPTED',
   'SCORED_TASK',
   'BOSS_DAMAGE', // Not used currently but kept to avoid validation errors
+  'GUILD_PROMPT',
+  'GUILD_JOINED_ACHIEVEMENT',
 ];
 
 const Schema = mongoose.Schema;