Always allow admin to delete exercises

[rolandgeider]

At the moment, it's only possible to delete exercises when a language is selected for which a translation exists, but it should be possible to always delete an exercise.

Currently the delete dialog allows the administrator to either delete the entire exercise or just the current translation. A possibility is to add a dropdown for all the available languages for the exercise, but just select the current one. Also, it should not be possible to delete English or the last available language.