Support multiple entries in a playlist for the same video

[dennis531]

The plugin currently only supports one entry for one video per playlist. This restriction is implemented in the database. On the other hand, Opencast playlists can contain multiple entires of the same video in one playlist. As we do not have a specific use case for this, we should not provide this function in the UI. Nevertheless, the plugin should be able to handle multiple entries of the same video in the database and display them to the user in order to avoid incompatibilities.

Related #955