Sunsetting: Remove config enforcement, and post-update hook to do so for existing sites

[adam-vessey]

GitHub Issue: #74 / Islandora-Devops/islandora-starter-site#2

• Other Relevant Links (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

What does this Pull Request do?

Introduces a (post-)update hook and documents its usage in the process of removing of this module.

What's new?

The module is removable without removing its config, with a new post-update hook to facilitate existing installations.

• Does this change require documentation to be updated? Probably?
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? Somewhat... ish? Has knock-on effects with other packages, but the update hook on its own does not really... it's more the other process of removing the module that gets into things.

How should this be tested?

Easiest might be to:

1. Spin up a "revived" instance of the standard islandora-playbook, to get a a base environment with islandora_defaults still present
   • Should be sufficient to just grab the old standard.yml into islandora-playbook's vars directory
   • Then, the usual: ISLANDORA_INSTALL_PROFILE=standard vagrant up
2. Perform a config export: drush config:export sync
3. Run through the "Deprecation/Removal Preparation" steps in the README.md
   • Before merge/release, it will be necessary to first add a repository entry to /var/www/html/drupal/composer.json, pointing at the repo containing the branch being merged here, and to specify it in lieu of ^3... so, something like: composer require "islandora/islandora_defaults:dev-fix/config-enforcement as 3.x-dev" as the example output of execution presently uses instead of the composer require "islandora/islandora_defaults:^3" which is in the main doc.
   • ... should do the trick.
4. Do another config export, but examine the diff: drush config:export sync --diff
   • Should just see the enforced statements targeting islandora_defaults going away, and entries from the core.extension changing according to the modules that were uninstalled (and possibly some re-ordering of properties just due to config doing config things).

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/committers

[nchiasson-dgi]

Followed the Test steps provided and produces the results expected. Islandora Defaults is successfully removed as well as islandora_defaults module enforcement within the config.