replace self-made sortable model with jazzband django-admin-sortable

[Bomme]

Description
I'm proposing to replace our implementation of a sortable model with the jazzband package django-admin-sortable

The idea came, after our linter discovered that the current code was broken and I tried to wrap my head around what the code is doing.

Deployment steps:

1. migrate

[alastair]

I think that we found that there was just one model that was sortable, right? Licenses is sortable but we can't find where that logic is used. In that case, I'd just add SortableMixin and the order field directly to the Forum model

[Bomme]

right, alternatively we could make it sortable in admin with adding LicenseAdmin(SortableAdmin).
What would happen if we remove the ordering from the License model (table)?

[alastair]

I'd double-check with @ffont if we need to order licenses, and if not we remove it.

[ffont]

I think sortable is only used in forums because, well, we wanted a custom ordering to display forums. But this is not needed for Licenses so no need to add it to them.