Split API URLs into more digestible bits.

[terjekv]

The splitting of views was a very good thing, and doing the same to URLs should be done. In doing this, there is also bound to be some reorganisation of the URLs as well. Suggested scopes:

urlpatterns = [
    path("", include(router.urls)),
    path("iam", include(iam.urls)),
    path("attachments", include(attachment.urls)),
    path("extensions", include(extension.urls)),
    path("resources", include(resource.urls)),
]

This will require a lot of rewriting of tests, which may be combined with some early work on #68 when applicable.