Enable metrics dashboard to have rearrangeable panels and save to backend

In order to have #27835 (closed), from the frontend side we must have:

• The removal / resorting of metrics triggers a change in Vuex store and gets saved to the backend
• rearrangePanelsAvailable is enabled

Original Thread

The following discussion from !16605 (merged) should be addressed:

• @mishunov started a discussion: (+3 comments)

  Since this method has not been implemented in this MR, I think we should not leave the placeholders. Means, @click should do nothing or simply be removed from the markup. Up to you do decide @mrincon ;)