Option to disable stack usage

You now can disable the usage of the stack. If set to ``false`` the stack and correspondening buttons are removed from the user interface.

Is path ``general/interface/stack/disable`` good named? For me it's ok but we have already quite some not that lucky naming in config. So some discussion before adding further ones perhaps isn't a bad idea. :-)

Eg. using ``gui`` instead of ``interface`` comes to mind…
What about `s/interface/admin/` and flipping the logic:

```yaml
admin:
  stack:
    enable: true  # Enable stack usage. Defaults to true
```