manually implementing postsave

[qadan]

Related issue: Islandora/documentation#1731

What does this Pull Request do?

Removes hook_post_action as a dependency, and manually implements the functionality it was providing.

The hook_post_action module likely won't be ported to Drupal 9. The functionality it provides is also being implemented in Drupal 9 at a later date. The intention of this pull request is to provide a stopgap between the loss of support for hook_post_action in Drupal 9 and our eventual ability to recreate this functionality when it's added into core in Drupal 9.1 or later.

What's new?

After an insert operation is taken for a Media, a registered shutdown function fires off file derivative actions for that Media.

How should this be tested?

We only need to ensure that there is no regression from #756, so the test case there will suffice.

Additional Notes:

The recommendation in this PR is to wait for entity events to be implemented in D9, but also, if we did the work to implement actions as mentioned in this comment, said future work would likely be enough to delay us having to migrate completely to entity events. But I think those entity events should be considered the ideal endgame.

Interested parties

@dannylamb likely as part of the ongoing D9 sprint https://github.com/orgs/Islandora/projects/3

[dannylamb]

I'm setting up the test case to put this through the paces. I'm also wondering if it will move this along in the Github project.

[dannylamb]

Just ran through a basic multifile media test. Works as expected. I was able to attach FITS files to a field_fits_file field on a new media type.

I had to do some refactoring in MediaSourceService and MediaSourceController, but that's totally unrelated to this and it can come in now. I'll make a subsequent PR for what I had to do.

[dannylamb]

Oh.. Travis is complaining about a doc comment @qadan . Fix that and I'll merge.