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

superset/views/key_value.py

@@ -0,0 +1,56 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+import simplejson as json
+from flask import request, Response
+from flask_appbuilder import expose
+from flask_appbuilder.security.decorators import has_access_api
+
+from superset import db, event_logger
+from superset.models import core as models
+from superset.typing import FlaskResponse
+from superset.utils import core as utils
+from superset.views.base import BaseSupersetView, json_error_response
+
+
+class KV(BaseSupersetView):
+
+    """Used for storing and retrieving key value pairs"""
+
+    @event_logger.log_this
+    @has_access_api
+    @expose("/store/", methods=["POST"])
+    def store(self) -> FlaskResponse:  # pylint: disable=no-self-use
+        try:
+            value = request.form.get("data")
+            obj = models.KeyValue(value=value)
+            db.session.add(obj)
+            db.session.commit()
+        except Exception as ex:  # pylint: disable=broad-except
+            return json_error_response(utils.error_msg_from_exception(ex))
+        return Response(json.dumps({"id": obj.id}), status=200)
+
+    @event_logger.log_this
+    @has_access_api
+    @expose("/<int:key_id>/", methods=["GET"])
+    def get_value(self, key_id: int) -> FlaskResponse:  # pylint: disable=no-self-use
+        try:
+            kv = db.session.query(models.KeyValue).filter_by(id=key_id).scalar()
+            if not kv:
+                return Response(status=404, content_type="text/plain")
+        except Exception as ex:  # pylint: disable=broad-except
+            return json_error_response(utils.error_msg_from_exception(ex))
+        return Response(kv.value, status=200, content_type="text/plain")