refactor: files{} -> challengeFiles[], and key -> fileKey (#43023)

* fix(client): fix client

* fix propType and add comment

* revert user.json prettification

* slight type refactor and payload correction

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* update ChallengeFile type imports

* add cypress test for code-storage

* update test and storage epic

* fix Shaun's tired brain's logic

* refactor with suggestions

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* update codeReset

* increate cypress timeout because firefox is slow

* remove unused import to make linter happy

* use focus on editor

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* use more specific seletor for cypress editor test

* account for silly null challengeFiles

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

client/gatsby-node.js

@@ -244,17 +244,11 @@ exports.createSchemaCustomization = ({ actions }) => {
   const { createTypes } = actions;
   const typeDefs = `
     type ChallengeNode implements Node {
-      files: ChallengeFile
+      challengeFiles: [FileContents]
       url: String
     }
-    type ChallengeFile {
-      indexcss: FileContents
-      indexhtml: FileContents
-      indexjs: FileContents
-      indexjsx: FileContents
-    }
     type FileContents {
-      key: String
+      fileKey: String
       ext: String
       name: String
       contents: String