Allow applications to register links in the admin interface

[Mathieu Poussin]

I think it would be interesting for custom applications to register links in various places in the admin area.

The typical example would be the admindocs module, instead of hardcoding it in the admin template, we could have something like

admin.register_link(title=_('Documentation'), url=reverse('...'), location='header_right', staff_only=True))

And a specific code in the template like :

{% for link in admin.links.header_right %}
<a href="{{ link.url }}">{{ link.title }}</a>
{% endfor %}

We could also add other locations (Like a new pane in the admin frontpage, etc.)

So any application could easily add its own links.

Currently, as far I a know, the only way to do this is to override the admin templates.