sandboxes: changing colours in Edit mode is a bit wierd

In the Sandbox UI, the user can click to Edit a sandbox, and change the colour.

As the colour is switched, the whole app changes in realtime to show users the consequence of the change. This is really cool, and really important feedback to the user!

But there are two major problems with it, which we'll need to resolve one day:

• The background modal is grey, so the user isn't seeing real colour, which makes it a bit harder to see the impact of your choice
• When you hit "Save", unless you happen to be editing the current sandbox, the colour will revert back - because of course you're editing a sandbox you're not looking at. This is correct, but weird! I think a nice solution would be to have three buttons: Cancel, Save, and Save & Switch (which saves your changes and switches into that sandbox). I don't think Save & Switch is desirable functionality, but I DO think that the presence of the button makes the behaviour much more predictable.

Raised out of #3588

Start from the Sandbox UI edit flow and reproduce changing a sandbox colour, including saving while viewing a different sandbox. Trace the modal and save behavior first. Done means colour previews are understandable in the modal and the save outcome is predictable, including the proposed Save & Switch option.