Integration with entity embed

The 2.x branch has the same issue regarding embedded entities as the 1.x branch. When on a Preview Link route and evaluating the access result of embedded entities, nothing in the result stack explicitly returns an AccessResultAllowed, so the embedded entity is not rendered.

Here's a new patch for the 2.1.x branch. I've removed the section that larowlan highlighted in #3, this is just checking that route entity has a preview link, but this is already implicitly true when we check that the route is a preview link route.

Found out that entity access might be determined and cached before hook_entity_embed_context_alter() has a chance to run, so we need to reset the entity type access cache.

Whoops, here's a fixed patch file.

Updated the patch to add a check for when getRouteName returns NULL (e.g. running in cron)

Please see #5 you can add additional entities (i.e the embedded ones) to the preview link.

Please let me know if this works for your use case.