index.js: change link to list of pulic XMPP providers for registrations

[yurtpage]

The registration process is the main pain for regular users (just imagine explaining this to your relatives).
The old compliance.conversations.im is not intended for users to pick a server. But this can be added to it with low efforts but this may be difficult to negotiate.
The Kaidan client uses the https://providers.xmpp.net/ as a categorized list of providers.
According to changelog the Converse used before some list of providers from the site #1206:

v4.0.1 (2018-09-19) Use https://compliance.conversations.im instead of dead link tot st https://xmpp.net

Unfortunately from the recommended A category only three servers doesn't require for a CAPTCHA which is annoying: https://jabber.fr, https://yax.im, https://xmpp.eath. And the BOSH is supported only by the jabber.fr. Basically we can just add it to the text.

We should ask other servers to enable the BOSH (and maybe disable the CAPTCHA). Anyway, it would be really good to put the conversejs.org as a default value (not just a placeholder) but also it's possible to add other domains into auto-completion to make it easier for user to type them (#2986). Can we add the conversejs.org to the list of public providers or you don't want to have many registrations from side clients?

BTW the conversations.im server has BOSH and is working but in the rating it has a low category because it has something disabled. I remember that asked its owner to fix that but he is too busy but also he wasn't happy to be used by anyone as a general server.

The compliance test checks if a server has XEP-0156: Discovering Alternative XMPP Connection Methods (HTTP)
https://compliance.conversations.im/test/xep0156/
So some of this servers with green check potentially may allow BOSH and allow registrations. If found a few from the list:
agnos.is
ari.lt
asozial.org

Ideally we need to have the list of such servers.

There are some other places that use the Compliance page that I didn't changed:
https://github.com/search?q=repo%3Aconversejs%2Fconverse.js%20compliance.conversations.im&type=code

Before submitting your request, please make sure the following conditions are met:

• Add a changelog entry for your change in CHANGES.md
• When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst
• Please add a test for your change. Tests can be run in the commandline
  with make check or you can run them in the browser by running make serve
  and then opening http://localhost:8000/tests.html.

[jcbrand]

Thanks for your pull request.

Anyway, it would be really good to put the conversejs.org as a default value

I agree it would be better from a user perspective, but I run that server and don't want the headaches and hassles of running a large public server. At least not at the moment.

On the topic of BOSH, it's not really that important anymore because we can now have websocket connections which are much better, so I don't consider BOSH support to be very important.

[yurtpage]

New usual users often confused that they need to register :) So we need to simplify this step for them as much as possible.