Add option to mark a model "immutable"

Hi,

In some cases, for important models, we don't want to accidentally make changes. Changes such as application config changes, adding or removing units, adding or removing applications, and worse of all, accidental destroy model.

Is it possible to add an option to mark a model "immutable" so it's locked from changes? With an option to unlock if one needs to make changes?

It'll also be useful to lock the controller model so it's not accidentally destroyed.