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>

utils/sort-files.js

@@ -1,5 +1,5 @@
 exports.toSortedArray = function toSortedArray(challengeFiles) {
-  const xs = Object.values(challengeFiles);
+  const xs = challengeFiles;
   // TODO: refactor this to use an ext array ['html', 'js', 'css'] and loop over
   // that.
   xs.sort((a, b) => {