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>

curriculum/schema/challengeSchema.js

@@ -6,7 +6,7 @@ const { challengeTypes } = require('../../client/utils/challenge-types');
 const slugRE = new RegExp('^[a-z0-9-]+$');
 
 const fileJoi = Joi.object().keys({
-  key: Joi.string(),
+  fileKey: Joi.string(),
   ext: Joi.string(),
   name: Joi.string(),
   editableRegionBoundaries: [Joi.array().items(Joi.number())],
@@ -37,12 +37,7 @@ const schema = Joi.object()
       then: Joi.string().allow(''),
       otherwise: Joi.string().required()
     }),
-    files: Joi.object().keys({
-      indexcss: fileJoi,
-      indexhtml: fileJoi,
-      indexjs: fileJoi,
-      indexjsx: fileJoi
-    }),
+    challengeFiles: Joi.array().items(fileJoi),
     guideUrl: Joi.string().uri({ scheme: 'https' }),
     helpCategory: Joi.valid(
       'JavaScript',
@@ -76,15 +71,7 @@ const schema = Joi.object()
         crossDomain: Joi.bool()
       })
     ),
-    solutions: Joi.array().items(
-      Joi.object().keys({
-        indexcss: fileJoi,
-        indexhtml: fileJoi,
-        indexjs: fileJoi,
-        indexjsx: fileJoi,
-        indexpy: fileJoi
-      })
-    ),
+    solutions: Joi.array().items(Joi.array().items(fileJoi)),
     superBlock: Joi.string().regex(slugRE),
     superOrder: Joi.number(),
     suborder: Joi.number(),