Move block preview template to wagtailcore

This allows us to use it in wagtail.blocks without depending on wagtail.admin

wagtail/admin/views/generic/preview.py

@@ -170,7 +170,7 @@ class PreviewRevision(View):
 
 @method_decorator(xframe_options_sameorigin_override, name="get")
 class StreamFieldBlockPreview(TemplateView):
-    template_name = "wagtailadmin/generic/streamfield_block_preview.html"
+    template_name = "wagtailcore/shared/block_preview.html"
     http_method_names = ("get",)
 
     @cached_property

wagtail/test/testapp/templates/tests/custom_block_preview.html

@@ -1,4 +1,4 @@
-{% extends "wagtailadmin/generic/streamfield_block_preview.html" %}
+{% extends "wagtailcore/shared/block_preview.html" %}
 {% load static %}
 
 {% block content %}

wagtail/test/testapp/templates/tests/static_block_preview.html

@@ -1,4 +1,4 @@
-{% extends "wagtailadmin/generic/streamfield_block_preview.html" %}
+{% extends "wagtailcore/shared/block_preview.html" %}
 {% load static %}
 {% block content %}
     <img src="{% static image_path %}" alt="{{ image_description }}" />