Adds final touches to custom fields across the network

[matt-bernhardt]

Why are these changes being introduced:

The AKDC site on our network has a set of custom fields added to its Post content type, which allow this content type to function as an event.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/lm-147

How does this address that need:

This defines a site-specific extension for the Mitlib Post plugin family, which will be enabled only on the AKDC site. This is the same approach which we've previously adopted for the Parent and News sites when those sites needed to extend the built-in post types.

Document any side effects to this change:

This doubles down on using the Post content type for this content, when we might really need to define an Events post type - but that will need to be a coordinated change after we launch the rebuilt site.

Recommended verification for this PR

• Log into the AKDC site, in the Custom Fields area of the admin dashboard, at /akdc/wp-admin/edit.php?post_type=acf-field-group
• Note that the only set of custom fields listed is shown under a "Sync available" tab
• Load the "Add New Post" form at /akdc/wp-admin/post-new.php and note that there is a "News and Events" field group visible
• You can also see the these fields used to render existing content by editing an existing post that has event information, like Post 2194: /akdc/wp-admin/post.php?post=2194&action=edit

Developer

Secrets

• No secrets are affected

Documentation

• No documentation changes are needed

Accessibility

• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)

Stakeholder approval

• Stakeholder approval is not needed

Dependencies

YES dependencies are updated

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• The changes have been verified
• The documentation has been updated or is unnecessary
• New dependencies are appropriate or there were no changes

[JPrevost]

This looks to do what it intends to do, and I agree we can revisit whether extending Posts or creating new types is better later.

Question regarding migration: did the existing test migrations for akdc bring in event data properly and this just allows us to add/edit/display that data or do we need to confirm that migrations will pull this data in now that we have support for it?

[matt-bernhardt]

One of the quirks here is that the custom fields and field groups are also included in the content export - so the fact that I missed these fields in code initially didn't mean that these fields were left out of the first import. They were just entirely in the content side of things.

With this PR, we have the option to remove those items from the content XML (although this isn't necessary). I intend to do this for the final import, FWIW.

[JPrevost]

@matt-bernhardt lets chat about what the effect of your last comment means in slack or zoom to make sure I understand it. I'm not blocking merging as this is working as intended, but I'd like to ensure I understand what risk we are assuming in the akdc site migrating differently than in the final migration than our last test migration if nothing else. Thanks!