[frontend] Make it possible to configure visibility of the help menu …

[tarunjangid]

…in the sidebar

What changes were proposed in this pull request?

• There was a request in our community to have a feature to remove the help tip - https://github.com/cloudera/hue/issues/3647

• Currently, the "Help" menu in the sidebar is visible to all the users.

• In this PR, a new configuration named is_help_menu_enabled is added to control the help menu.

• If this configuration is set to False, we cannot see the Help menu as below:

• If the configuration is set to True (the default value), the Help menu is visible as usual:

How was this patch tested?

• manually tested in my local setup

Please review Hue Contributing Guide before opening a pull request.