add button to hibernate and wake-up cluster

[petersutter]

What this PR does / why we need it:
Added ability to to hibernate (and wake-up) clusters to save costs when they are temporarily not needed.

Which issue(s) this PR fixes:
Fixes #28

Special notes for your reviewer:

Release note:

You can now hibernate and wake-up clusters on the cluster details page

[grolu]

Now that the user can hibernate in the UI via a button click, the fact that hibernated; reconcile processing state shows a checkmark with a circular process, is even more confusing. We should discuss what to show to the user in this case. Maybe we could only show zzz without a circle, as the user probably does not care about reconciling of hibernated clusters. What do you think?

[petersutter]

@rfranzke is it crucial for an operator to see if the reconcile is running for a hibernated shoot? If yes I would suggest to show the progress circle with zzz
but in any case, I agree with @grolu that showing the checkmark with a progress circle in this case is confusing

[rfranzke]

If it's a reconciliation operation I guess we can skip it in case the shoot is hibernated, however, any error that may occur or any other operation should be visible.

[grolu]

lgtm