Refactor bucket delete and bucket policy (#5580)

This commit adds the bucket delete and bucket policy functionalities
to the browser.

Part of rewriting the browser code to follow best practices and
guidelines of React (issues #5409 and #5410)

The backend code has been modified by @krishnasrinivas to prevent
issue #4498 from occuring. The relevant changes have been made to the
code according to the latest commit and the unit tests in the backend.
This commit also addresses issue #5449.

browser/app/js/buckets/BucketPolicyModal.js

@@ -0,0 +1,61 @@
+/*
+ * Minio Cloud Storage (C) 2018 Minio, Inc.
+ *
+ * Licensed 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 React from "react"
+import { connect } from "react-redux"
+import { Modal, ModalHeader } from "react-bootstrap"
+import * as actionsBuckets from "./actions"
+import PolicyInput from "./PolicyInput"
+import Policy from "./Policy"
+
+export const BucketPolicyModal = ({ showBucketPolicy, currentBucket, hideBucketPolicy, policies }) => {
+  return (
+    <Modal className="modal-policy"
+            animation={ false }
+            show={ showBucketPolicy }
+            onHide={ hideBucketPolicy }
+    >
+      <ModalHeader>
+        Bucket Policy (
+        { currentBucket })
+        <button className="close close-alt" onClick={ hideBucketPolicy }>
+          <span>×</span>
+        </button>
+      </ModalHeader>
+      <div className="pm-body">
+        <PolicyInput />
+        { policies.map((policy, i) => <Policy key={ i } prefix={ policy.prefix } policy={ policy.policy } />
+          ) }
+      </div>
+    </Modal>
+  )
+}
+
+const mapStateToProps = state => {
+  return {
+    currentBucket: state.buckets.currentBucket,
+    showBucketPolicy: state.buckets.showBucketPolicy,
+    policies: state.buckets.policies
+  }
+}
+
+const mapDispatchToProps = dispatch => {
+  return {
+    hideBucketPolicy: () => dispatch(actionsBuckets.hideBucketPolicy())
+  }
+}
+
+export default connect(mapStateToProps, mapDispatchToProps)(BucketPolicyModal)