Make seriesmanagement work properly with or without capabilities to create or import series.

[justusdieckmann]

Removes the defineseriesforcourse capability, since it wasn't coherently used.

Now requires manageseriesforcourse for manageseries.php, and show import series / create new series buttons depending on respective capabilities.

Give teachers manageseriesforcourse and createnewseries capabilities by default on new installations.