Implemented new Achievement and Badge: Joined a Challenge (Fixes #8613) (#8761)

* Added image

* Added new achievement to user schema

* Added new achievement to content

* Added new achievement to libs

* Added achievement text to locale

* Added achievement to notification model and controller

* Grant achievement on joining or creating first challenge

* Added achievement to modal template

* Compiled new sprites

* Added integration tests

* Fix linting error

website/server/models/user/schema.js

@@ -115,6 +115,7 @@ let schema = new Schema({
     getwell: Number,
     royallyLoyal: Boolean,
     joinedGuild: Boolean,
+    joinedChallenge: Boolean,
   },
 
   backer: {