Add GraphQL mutation to update a release

What does this MR do?

Adds a GraphQL mutation to update an existing release.

This is the GraphQL equivalent of the PUT /projects/:id/releases/:tag_name API endpoint. It follows the same permission model (only Developers or higher are allowed to update releases.)

Example query/response

Query

mutation ($input: ReleaseUpdateInput!) {
  releaseUpdate(input: $input) {
    release {
      name
      tagName
      description
      releasedAt
      milestones {
        nodes {
          title
        }
      }
    }
    errors
  }
}

Variables:

{
  "input": {
    "projectPath": "root/release-test",
    "tagName": "v5.34",
    "name": "Updated name",
    "description": "Updated description",
    "releasedAt": "2018-12-10",
    "milestones": ["12.3", "12.4"]
  }
}

Response:

{
  "data": {
    "releaseUpdate": {
      "release": {
        "name": "Updated name",
        "tagName": "v5.34",
        "description": "Updated description",
        "releasedAt": "2018-12-10T05:00:00Z",
        "milestones": {
          "nodes": [
            {
              "title": "12.4"
            },
            {
              "title": "12.3"
            }
          ]
        }
      },
      "errors": []
    }
  }
}

When the user doesn't have permission

Response:

{
  "data": {
    "releaseUpdate": null
  },
  "errors": [
    {
      "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "releaseUpdate"
      ]
    }
  ]
}

An example of a response with a top-level error

{
  "data": {
    "releaseUpdate": null
  },
  "errors": [
    {
      "message": "if the releasedAt argument is provided, it cannot be null",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "releaseUpdate"
      ]
    }
  ]
}

An example of a response with an error-as-data

{
  "data": {
    "releaseUpdate": {
      "release": null,
      "errors": [
        "Tag does not exist"
      ]
    }
  }
}