Rewrite Desktop Wallet (JS) tutorial

[mDuo13]

As noted in #3329, the "Build a Desktop Wallet in JavaScript" tutorial is not quite up to our standards. Specifically:

1. The code blocks in the tutorial should use {% code-snippet ... /%} instead of copy-pasting code from the samples into the file (since the copy-pasted code can get out of sync with the actual code samples)
2. The language used in the tutorial, including in the comments in the code sample, does not quite fit our style guide, and it can be made a lot more concise, avoid future tense, and so on.
3. It would be nice to update the code style to EJS (currently it's mostly CJS)
4. Currently, the sample code is split into different "snapshots" per step, but the "helpers" files are shared across snapshots, which means that their import paths are different than if you were following along the tutorial and making one evolving app. It would be nice to resolve this tension somehow.