Support "rebalancer_mode" and "rebalancer" options on web UI level

[RunsFor]

There is new functionality in the latest vshard versions - options, which controls the rebalancer behaviour.

In the #2186 it's been supported on backend level. Now it would be great to reflect these changes in the web UI.

I.e. it would be great to be able to:

• see/set rebalancer_mode option globally. It can be one of 3 options - off, manual or auto
• see/set rebalancer option for each replicaset/storage

[yngvar-antonsson]

I think we should alsp show rebalancer_enabled field on the main cluster page

[yngvar-antonsson]

1. rebalancer_mode on the same level as failover
   Graphql request should be the same as the "change replicaset weight request"
2. rebalancer (true, false, null) on edit replicaset page and somewhere on edit server page, which we don't have.

mutation($replicasets: [EditReplicasetInput]) {
  cluster {
    edit_topology(replicasets: $replicasets) {
      replicasets {
        uuid
        rebalancer
      }
    }
  }
}

----

replicasets = {{
    uuid = replicaset_uuid,
    rebalancer = true,
}}

mutation($servers: [EditServerInput]) {
  cluster {
    edit_topology(servers: $servers){
      servers {
        uuid
        rebalancer
      }
    }
  }
}
---
 servers = {
    {uuid = uuid, rebalancer = true},
}

3. On the main page, near the instance nave we should see a pictogram of rebalancer enabled.

{
   servers {
     uri
     boxinfo {
       vshard_storage {
       rebalancer_enabled
     }
   }
 }
}