UI Subsystems Customization

# Adding a way to show/hide RHQ subsystems in UI.

Current UI is overcomplicated. RHQ does lots of things and
different users use it for different purposes. Although many
subsystems are interconnected and may depend on each other
(e.g. metrics and alerts). This is not the case for all of them
and in the sake of simplicity some of them could be hidden in
UI if user wants to.

This PR allows to hide following subsystems/features in UI:
 * content
 * reports
 * administration
 * events
 * operations
 * alerts
 * config
 * drift

> **NOTE:** This change is only about hiding it in UI, the subsystems are still working and can be accessed via REST, CLI or direct URL.

The subsystems can be shown/hidden on the user page
`${host}/coregui/#Administration/Security/Users/${userId}`
and this information is saved to the user preferences. 

It is orthogonal to the access rights that are specified for 
roles. Each user may decide what to hide to make the UI
cleaner for him/her.