Add Editor component to the Mobile IDE View

[ghalestrilo]

This PR mounts the <Editor /> component on the Mobile IDE View (/mobile), and places a button for exiting the mobile view.

Functionality is only partially implement, as expected: running the code will require a new overlay component.

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[catarak]

You should import CloseIcon from client/common/icons.jsx, and wrap it in a <Link> or <button> depending on what you want it to do. And then, because this icon has no text but performs an action, it should have the attribute aria-label="close header" (or whatever the alt text should be.

[ghalestrilo]

Ok, will do that!

[catarak]

Since we're using styled components, I think it makes sense to not use any inline CSS like this! You also shouldn't need aria-label or aria-hidden on the CloseIcon, as aria-label should be added to the <Link>, see https://www.scottohara.me/blog/2019/05/22/contextual-images-svgs-and-a11y.html

[ghalestrilo]

Makes sense. Just updated this!