'Back to Editor' links to the last opened project (Fixes #2175)

[lindapaiste]

Fixes #2175

Changes:

• "Back to Editor" links to the URL of the active sketch based on the Redux state.
• If no sketch is opened, it links to the root / as before.

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

---

I do have a question with regards to the special handling of URLs for p5 examples, which is applied in the SketchList. Should that be applied here as well? Both versions of the URL work. If we want to prefer the "pretty" version then we can extract that logic into a utility function and use it in multiple places.

p5.js-web-editor/client/modules/IDE/components/SketchList.jsx

Lines 276 to 279 in dd433b7

	let url = `/${username}/sketches/${sketch.id}`;
	if (username === 'p5') {
	url = `/${username}/sketches/${slugify(sketch.name, '_')}`;
	}