Add option to delete ensembles/experiments

[oyvindeide]

Currently there is no way for users to delete ensembles and experiments. Not having the option to delete also makes it harder for us to make ensembles immutable, as the number will just grow, taking up more and more space.

Perhaps we should start with just being able to delete experiments, as deleting individual ensembles is more challenging, as we then need to consider what to do with parent ensembles, and so forth.