Finished first working version of a Drupal 8 port for htmlpurifier

Use settings form with a textarea to edit configuration in YAML

Reformatted hook_help and replaced the deprecated \Drupal::url function.

Validated the Composer.json

Nitpicks

* '8.x-1.x' of https://github.com/heddn/htmlpurifier:
  clean up the definition of a cache backend for htmlpurifier
  got the cache backend to work for htmlpurifier

First module upgrader suggestions.

Conflicts:
	htmlpurifier.module

Converted hook_help().