refactor: Break up superset/views/core.py (#10078)

* Remove unreferenced function from views/core.py

* Remove excess constants from views/core.py

* Extract CssTemplate-related views to their own file from core.py

* Remove duplicate constant declaration and make the constant less racist

* Move free-floating functions in views/core.py to views/utils.py

* Move AccessRequestsModelView out of views/core.py into its own module

* Move health checks and KV ModelView out of core.py and into their own modules

* Move R model view to its own module

* Move after-request header setting to views/base.py from views/core.py

* black

* mypy

* isort

* Fix reference to imported app

* pylint

* Fix some imports

* Add some missing view imports

* Fix a missing import

tests/core_tests.py

@@ -37,6 +37,7 @@ from unittest import mock, skipUnless
 import pandas as pd
 import sqlalchemy as sqla
 
+import superset.views.utils
 from tests.test_app import app
 from superset import (
     dataframe,
@@ -1106,7 +1107,7 @@ class CoreTests(SupersetTestCase):
         self.assertIsInstance(serialized_payload, str)
 
         query_mock = mock.Mock()
-        deserialized_payload = views._deserialize_results_payload(
+        deserialized_payload = superset.views.utils._deserialize_results_payload(
             serialized_payload, query_mock, use_new_deserialization
         )
 
@@ -1159,7 +1160,7 @@ class CoreTests(SupersetTestCase):
             query_mock = mock.Mock()
             query_mock.database.db_engine_spec.expand_data = expand_data
 
-            deserialized_payload = views._deserialize_results_payload(
+            deserialized_payload = superset.views.utils._deserialize_results_payload(
                 serialized_payload, query_mock, use_new_deserialization
             )
             df = results.to_pandas_df()