View API Output links wrong when viewed with language prefix

@perraudeau thank you for your contribution!

I've looked at your code, and my conclusion is that I'm still going to create a PR with the proposed resolution: change getAdminBaseUrl(). Because:

• For the basic case (without domain module), I believe the issue description is enough. It's simpler.
• The fact that getAdminBaseUrl() has a language prefix, is unintended behavior that is causing problems elsewhere. So it should be fixed anyway.

This still needs internal testing from our side, and a fix for a strange issue, so it's not going to get merged yet.

--

In the meantime, I'm curious to your issue:

• Can you test whether the new PR resolves your issue? (I'm guessing it won't, because the domain module adds complications)
• If not: can you describe precisely
  • what API urls you see, without your patch (and where - if not in /admin/content
  • and what they should be?

Then we can use this for PHPUnit tests, which should be added before a solution gets committed. (If you can contribute a precise description for an example setup with domain module, if not PHPUnit tests... that helps.)

---

And then, we can also discuss your solution. It may not be necessary to go into detail right now, but I'll still describe my review of your code. I've seen:

• Fundamentally you're replacing $entity->toUrl() with Url::fromRoute(). I don't have a fundamental problem with that except...
• You're adding a route for /ce-api/node/{node}. I am hoping that this won't be necessary in the end. (Maybe we can get $entity->toUrl() to work in #3346436: Introduce API for getting the /ce-api URL, and make sure it works with the Domain module at the same time?)
• With this, you are adding support of URLs like DOMAIN/LANGCODE/ce-api/node/..., which are not supported without your patch (we only support DOMAIN/ce-api/LANGCODE/node/.... We can discuss this if it's really necessary, but I am hoping we can do without this. At any rate, the str_contains() change in BackendApiRequest.php is too broad. We do not want to match + change /ce-api/ just anywhere in the path.

I have tried the tests I wrote, with your solution. They run fine. (Except they fail on

1. links are with the backend domain before your patch, and without the backend domain after. (So: DOMAIN/ce-api/... changes to /ce-api/....) I don't think that's a problem and we could change the tests if needed.
2. the same strange/separate issue as with my solution

)
--

The "strange issue" which is still failing the tests:

I have enabled the views module in PHPUnit tests, because without views I don't get an original + translated node in the same list, in /admin/content.

But now... a separate test is failing: the json-ld-schema breadcrumbs on /node/1 seem to be containing the backend URL, when views is enabled!

Even though that's a separate issue, we should still fix it and we can't commit something with failing tests. I'll look into it later.

MR #82 works. It's a simple fix... combined with a lot of test additions / workarounds. For review now.

@perraudeau if this gets set to "Fixed" after review, and your issue with the domain module persists, feel free to open a new ticket. As said: I'd love to know which "wrong" links are fixed by your change, and what the "right / fixed" links are.