Allow automatically selecting repository storage on move

What does this MR do?

gitaly#3209 (closed)

When moving project repositories, if destination_storage_name is not provided, use the same logic as Project to assign it. These are currently weighted by an admin configuration page.

Manual Testing

Setup gdk.yml:

praefect:
  enabled: true
gitaly:
  storage_count: 4

Rejigger gdk:

$ gdk reconfigre
$ gdk restart

Check where the project is currently stored:

$ curl -H "PRIVATE_TOKEN: XXXX" http://gdk.test:3000/api/v4/projects/19 | jq
{
  "id": 19,
...
  "repository_storage": "default",
...
}

Reconfigure repository weights:

Initiate the move:

$ curl -X POST -H "PRIVATE_TOKEN: XXXX" -H "Content-Type: application/json" -d '{}' http://gdk.test:3000/api/v4/projects/19/repository_storage_moves | jq
{
  "id": 17,
  "created_at": "2020-10-16T02:28:15.149Z",
  "state": "scheduled",
  "source_storage_name": "default",
  "destination_storage_name": "gitaly-2",
  "project": {
    "id": 19,
    "description": "",
    "name": "Gitaly",
    "name_with_namespace": "Administrator / Gitaly",
    "path": "gitaly",
    "path_with_namespace": "root/gitaly",
    "created_at": "2020-06-29T22:18:29.342Z"
  }
}

gitaly-2 was chosen 🎉

Try again:

$ curl -X POST -H "PRIVATE_TOKEN: XXXX" -H "Content-Type: application/json" -d '{}' http://gdk.test:3000/api/v4/projects/19/repository_storage_moves | jq
{
  "id": 18,
  "created_at": "2020-10-16T02:29:21.260Z",
  "state": "scheduled",
  "source_storage_name": "gitaly-2",
  "destination_storage_name": "gitaly-3",
  "project": {
    "id": 19,
    "description": "",
    "name": "Gitaly",
    "name_with_namespace": "Administrator / Gitaly",
    "path": "gitaly",
    "path_with_namespace": "root/gitaly",
    "created_at": "2020-06-29T22:18:29.342Z"
  }
}

gitaly-3 was chosen 🎉 🎉

Does this MR meet the acceptance criteria?

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• Changelog entry
• Documentation (if required)
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• Style guides
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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