[EDU-765] Update jwt authentication tutorial

[fliptopbox]

Description

As part of the stocktaking of all tutorials (see: https://ably.atlassian.net/browse/EDU-765)
This change repated to the updated tutorial PR ably/tutorials#104
we found inconsistencies, and failing code samples, for the most visited tutorial.

This PR is a QUICK-FIX the following:

• the Textile instructions do not match the Github repo code snippet (see: https://github.com/ably/tutorials/tree/jwt-authentication-nodejs-fix) that fixed this mismatch
• the input form on the demo is not used to set the clientId and is redundant, the fix corrects this redundancy
• the JavaScript ES6 updates and styling changes for: use of const, and spread operator, and fat arrow functions.

Related work:

• Update the live demo CSS for docs on Website page Website PR
• Update the docs tutorial, to use the update screenshot and updated JavaScript docs tutorial PR
• Update the tutorials source code, and replace out-of-date ably logo. Source code PR

[fliptopbox]

@tbedford & @marklewin this is my very first docs contribution, so i am tending on the side of caution for this PR.
Please note this has a complimentory PR on the docs repo. See ably/tutorials#104

[marklewin]

Looks good. Love the fancy new dialog 👏 I've made a few suggestions to help make the instructions a bit clearer and fix a couple of grammar/typo things.

[marklewin]

Suggest making the opening of the browser console a step in its own right, otherwise there's not really any point in having the console.log() statements.

[marklewin]

Great start. A few typos and rewording suggestions and also for improving the live demo.

[tbedford]

I've run through and tested. LGTM!