Disable cluster delete button in garden namespace

[petersutter]

The garden namespace is only accessible to operators. However, to prevent any deletions by mistake of shoots in this reserved namespace the delete button should be disabled.
Optionally introduce a config flag to enable/disable this behaviour of the UI

[mvladev]

garden namespace is not special. I could name it garden-system or something similar and install my components there.

[petersutter]

currently we have several places in the dashboard where the garden namespace is hardcoded. In addition, even the gardener itself has a GardenNamespace constant.
Currently we live with this assumption until there is a need to make it more dynamic, but then also gardener has to adapt.

[grolu]

We discussed that we could (instead of hardcoding Garden namespace) also disable critical actions (e.g. delete hibernate, etc.) for clusters with purpose INFRA