[Meta] Offer a revisions tab for all entities

Rerolled #21 and moved to an MR
Hiding patches

larowlan → changed the visibility of the branch 2452523-meta-offer-a to hidden.

Updating title to reflect this isn't a meta, there's code.

Will continue with tests tomorrow

Updated issue summary
This now includes tests and is ready for review
Testing on a client project its working nice 🙌

I just tested out the issue fork and it works like a charm - only found a problem with custom block content entities. The revision overview seems to work as expected (with the radio buttons to compare selected revisions), but when clicking Compare selected revisions, I get this error:

Drupal\Core\Entity\Exception\UndefinedLinkTemplateException: No link template 'revision' found for the 'block_content' entity type in Drupal\Core\Entity\EntityBase->toUrl() (line 211 of core/lib/Drupal/Core/Entity/EntityBase.php).

This looks great, thanks to everyone who worked on it.

@larowlan do you think we should add a test case for media revisions as well, so all core entity types with the generic revision UI are supported?

@hctom can you provide more info about your Drupal version and any other steps to reproduce it? The test coverage suggests that block content is working so perhaps you have a patch or custom/contrib module that is interfering?

Can do for media
I did have that issue with block content before one of the later pushes, so perhaps @hctom was testing an earlier version.
I fixed it whilst adding the tests.

I used the latest patch on a Drupal 10.3.6 install with nothing fancy in there. It's just a block content entity bundle with a custom bundle class.
I also updated the patch with the latest changes now, but the errors persists - and as far as I see, the tests do not show any problems, because they only test the revision overview currently and not the actual revision comparison, where this error occurres.

I was able to reproduce @hctom's results
Expanded the tests and fixed the issue

Back to needs review

Just checked the new changes by larowlan and I can confirm: comparing revisions of block content entities is now working as expected. Thank you!

Rebase is failing tests

Back to green, would be good to get another review on this

Exported MR into patch file.

Responding to acbramley's comment, I took a first pass at de-duplicating the entity form and node form. The node form is now a subclass of the entity form, with a few selective overrides as needed to preserve the former behavior. Also resolved the differences in the table layouts, so that the entity form now looks and acts like the node form did before these changes.