Added a new "upload" permission that allows user to launch import and view
their own imports (#230)

New "upload" permission
^^^^^^^^^^^^^^^^^^^^^^^

This new permission is helpful if you want to give upload/import rights
to some users, but don't want them to be able to manage the library as a whole:
although there are no controls yet for managing library in the fron-end,
subsequent release will introduce management interfaces for artists, files,
etc.

Because of that, users with the "library" permission will have much more power,
and will also be able to remove content from the platform. On the other hand,
users with the "upload" permission will only have the ability to add new
content.

Also, this release also includes a new feature called "default permissions":
those are permissions that are granted to every users on the platform.
On public/open instances, this will play well with the "upload" permission
since everyone will be able to contribute to the instance library without
an admin giving the permission to every single user.