Make default topbar menus configurable by Django Admin

[giohappy]

The default menus in the catalog's topbar (All Resources, Datasets, Maps, Documents, Geostories, and Dashboards) are set in code by the get_base_left_topbar_menu and get_base_right_topbar_menu template tags.
Any custom menu is placed beside these fixed menus.

Often we receive requests to modify the default menus, for example hiding items, changing the labels, or placing custom items in between them.
This requires overriding the default template tags and the template snippets that render them.

We want to manage the default templates as Menu models, that can be customized by the Django admin.
Two additional flags will be added to the Menu and the MenuItem model to specify if a menu should be restricted to admin users and if it must be hidden from mobile agents.

The default menu items will be created with JSON fixtures which are loaded at installation time, along with the other predefined fixtures.
A project can provide a custom fixture that overrides the default one.