Drupal 8/9/10 Compatibility

Updating title to include Drupal 10.

I've not tested the zendesk integration. The module crashes on Drupal 9.5 because of the require 'vendor/autoload.php' at the top of the .module file.

A few minor nits (some of these might be on the main branch rather than the MR!)

- missing config schema file
- typos in module descriptions
- some info files have paths instead of route names for 'configuration' property
- admin UI should be under Web Services rather than System
- there's an empty folder zendesk_tickets with a .gitkeep file
- zendesk_users module invents hooks but doesn't document them
- zendesk_no_permission_page doesn't make it clear whether a path or a route name is required

Committed, with a few small tweaks.

I'll fix the other things in follow-ups. Thanks everyone!

Since this is a complete rewrite of 7.x-2.x, it should have a new version number, and so we might as well move to semver, so I've made a new 3.0.x branch, currently identical to 8.x-1.x.

Automatically closed - issue fixed for 2 weeks with no activity.