Generate the 404 page using React on the frontend.

[lindapaiste]

Fixes #2159
Fixes #2256

Done:

• Copy the HTML and CSS from the server 404 page to the frontend.
• Support translating the page texts (needs the translations).
• Fetch a random sketch from user p5.
• Display sketch using existing EmbedFrame component.
• Disable the portions of the EmbedFrame code for sending errors to the IDE console from when it is used on the 404 page (via a new sendMessages prop).
• Adjust the canvas size to fill the area below the message only.
• Add a link to the sketch.
• Show the toolbar at the top of the 404 page.
• Remove malformed assets URL replacement. (Improper loading of static files on 404 pages #2256)
• Add a catch-all react-router route to show the 404 page when no route matches.
• Change the catch-all server route to return the renderIndex() content with a 404 status.
• Cleanup server "object exists" checks with async/await syntax.
• Show the frontend 404 page when a matching user/collection/sketch object is not found by passing a variable window.__SERVER_404__ from server to client.

To Do:

• Figure out the correct settings for the textOutput gridOutput and basePath props.
• (maybe?) Create an API endpoint to return a single random sketch, instead of fetching all.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[lindapaiste]

@raclim I've got this to a point where it's working and I feel that it's a major improvement. Let me now if you have any questions about the code or any other feedback.

We will eventually want to add translations of the 404 page text for all languages.

There are some minor TODOs remaining. The missing env/config vars "PREVIEW_SCRIPTS_URL" and "EDITOR_URL" cause warnings in the console but no other problems. From what I can tell these vars are used for dispatching messages from the embedded sketch to the editor so that they can be displayed in the console. That behavior is not needed here. Ideally I would figure out how to avoid executing those parts of the code. done.

[lindapaiste]

@raclim I removed the refactoring of userExists etc. to make this clearer. I will submit those changes in a separate PR.

[lindapaiste]

ugh I need to fix the import of the Nav now that we refactored it and moved things around.

[raclim]

Thanks so much for keeping up and working on all of these different components and changes so far!

[PiyushChandra17]

Nicely Done 👍