Configurable list of bookmarks in sidebar menu

[jorge-leon]

Is your feature request related to a problem? Please describe.

• When integrating django-helpdesk into an existing Django project one can 'enter' the helpdesk, but not 'leave' -
  navigate back to other apps in the project.
• In a helpdesk environment it is helpful provide the personnel with links to related applications or sites.

Describe the solution you'd like

A configurable list of "bookmarks" in the sidebar menu, which is clearly separated from the django-helpdesk menu entries.
This bookmarks section must not show, when no bookmarks are configured.

Describe alternatives you've considered

None

Additional context

PR is following