New Fleets & Instances UI

[jvstme]

Problem

It is not possible to see a list of all fleets or all instances. It is not possible to manage fleets and instances.

Solution

Implement three pages with management controls: Fleets list, Fleet details, Instances list.
Details: https://docs.google.com/document/d/1_7Kc8bNUyYRhu29jpuLphUY4T0oF79h5AaKwoKgx-eM

This issue can be done in multiple stages. One way to split into stages:

• [UI] It's not possible to manage fleets now #2042
• Add a basic Instances list page - as described in the document, but without instance deletion and links to fleet details
• Update the Fleets list page as described in the document, but without links to fleet details
• Implement instance deletion on the Instances list page
• Add fleet details page and links to it from Fleets list and Instances list