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/__tests__/BucketDropdown.test.js

@@ -0,0 +1,62 @@
+/*
+ * 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 { shallow, mount } from "enzyme"
+import { BucketDropdown } from "../BucketDropdown"
+
+describe("BucketDropdown", () => {
+  it("should render without crashing", () => {
+    shallow(<BucketDropdown />)
+  })
+
+  it("should call toggleDropdown on dropdown toggle", () => {
+    const spy = jest.spyOn(BucketDropdown.prototype, 'toggleDropdown')
+    const wrapper = shallow(
+      <BucketDropdown />
+    )
+    wrapper
+      .find("Uncontrolled(Dropdown)")
+      .simulate("toggle")
+    expect(spy).toHaveBeenCalled()
+    spy.mockReset()
+    spy.mockRestore()
+  })
+
+  it("should call showBucketPolicy when Edit Policy link is clicked", () => {
+    const showBucketPolicy = jest.fn()
+    const wrapper = shallow(
+      <BucketDropdown showBucketPolicy={showBucketPolicy} />
+    )
+    wrapper
+      .find("li a")
+      .at(0)
+      .simulate("click", { stopPropagation: jest.fn() })
+    expect(showBucketPolicy).toHaveBeenCalled()
+  })
+
+  it("should call deleteBucket when Delete link is clicked", () => {
+    const deleteBucket = jest.fn()
+    const wrapper = shallow(
+      <BucketDropdown bucket={"test"} deleteBucket={deleteBucket} />
+    )
+    wrapper
+      .find("li a")
+      .at(1)
+      .simulate("click", { stopPropagation: jest.fn() })
+    expect(deleteBucket).toHaveBeenCalledWith("test")
+  })
+})