Remove content from public facing Examine indexes when it is made protected

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

The public facing Examine indexes (i.e. ExternalIndex) filters out protected content when indexing on publish and when rebuilding (it happens here). However, if content is made protected subsequently to a publish or index rebuild, the content remains in the indexes.

This PR ensures that content is removed from the applicable indexes when public access protection is applied.

Limitations

We have to leverage the public access cache refreshing notification to implement this, because we need to perform this task on every single server in a load balanced setup. Unfortunately, these notifications have been implemented rather sparsely, which limits what we can do with them.

At this time we can only remove content from the indexes when public access is added to content. We can't add content to the indexes when public access is removed from content.

To overcome this limitations, editors must republish the entire structure of the previously protected content after removing public access restrictions.

Testing this PR

You should probably start by rebuilding the ExternalIndex, to ensure you've got an accurate starting point.

You also need a member group (members optional) to perform public access configuration.

1. Create a structure of published content (i.e. a parent and a child)
2. Verify that the ExternalIndex contains all the content.
3. Apply public access restrictions at the top level of the structure.
4. Verify that the ExternalIndex no longer contains any item in the structure.
5. Remove the public access restrictions again.
6. Republish the entire structure (publish with descendants at top level).
7. Verify that the ExternalIndex once more contains all the content.

[Zeegaan]

Looks good, tests good! 💪
I'll merge once the merge-conflict is resolved 😁

[kjac]

@Zeegaan conflicts are resolved 👍

[Zeegaan]

Great, merging! 🚀