MetatagFirehose::formElement() should pass current entity explicitly to metatag_get_default_tags()

Problem/Motivation

If you create e.g. a node through a non-standard route, for example in combination with the group.module, then the metatag defaults aren't filled out correctly based on the node type.

That's because it calls metatag_get_default_tags() without passing the current entity to it explicitly, so that has to fall back to the defaults from the route, which only supports the default route.

It's also slower, especially for new forms, as it has to create a new entity just to figure out the entity type/bundle.

Proposed resolution

Change it to $default_tags = metatag_get_default_tags($items->getEntity());.

In some really weird cases, this *could* result in a problem if $items->getEntity() is actually not the node, e.g. if you add metatags through a paragraph to nodes, but that seems like a super-weird thing that shouldn't actually work :)

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet