[SPARK-9724] [WEB UI] Avoid unnecessary redirects in the Spark Web UI.

Author: Rohit Agarwal <rohita@qubole.com>

Closes #8014 from mindprince/SPARK-9724 and squashes the following commits:

a7af5ff [Rohit Agarwal] [SPARK-9724] [WEB UI] Inline attachPrefix and attachPrefixForRedirect. Fix logic of attachPrefix
8a977cd [Rohit Agarwal] [SPARK-9724] [WEB UI] Address review comments: Remove unneeded code, update scaladoc.
b257844 [Rohit Agarwal] [SPARK-9724] [WEB UI] Avoid unnecessary redirects in the Spark Web UI.