User dropdown menu for impersonation (only as superuser)

[infinitewarp]

As an authenticated superuser, I want to see a dropdown menu with a list of all available users so that I can choose one to impersonate and see data as that user.

https://redhat.invisionapp.com/share/UFI078IZVAW#/screens/296433282

Acceptance Criteria

• Verify superuser sees a user selection menu in the header.
• Verify regular users do not see this menu.
• Verify that the menu contains a list of all users in the system.
• Verify that selecting a user reloads the displayed data on the page with the appropriate user_id filter included on all of them.
• Verify that an issue for tracking integration tests exists and is referenced here.

Assumptions and Questions

• What exactly do we display in the menu? The email address?
• This does not necessarily include the "View Users" modal that includes more detailed information.
• The content of this menu is powered by the API defined in API to see list of users (as superuser only) cloudigrade#279
• Examples:
  • /api/v1/report/summary?start_date=2018-01-01&end_date=2018-02-02 without user filter
  • /api/v1/report/summary?start_date=2018-01-01&end_date=2018-02-02&user_id=5 with user filter