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.

cmd/web-handlers_test.go

@@ -1385,7 +1385,8 @@ func testWebListAllBucketPoliciesHandler(obj ObjectLayer, instanceType string, t
 	}
 
 	testCaseResult1 := []BucketAccessPolicy{{
-		Prefix: bucketName + "/hello*",
+		Bucket: bucketName,
+		Prefix: "hello",
 		Policy: policy.BucketPolicyReadWrite,
 	}}
 	testCases := []struct {