WIP: Add Release data to GraphQL endpoint

What does this MR do?

Adds all data currently available in our Release API to our GraphQL endpoint.

API shape

The shape of the GraphQL response mimics the shape of our existing Release REST API.

Permissions

Permissions line up with the existing project-level Release permissions.

In particular, if the user only has Guest permissions, sources will return [] and commit will return null. (See example response below.)

Feature flag

This new data is hidden behind a graphql_release_data feature flag, which is disabled by default.

Example query/response

Query:

{
  project(fullPath: "root/release-test") {
    release(tagName: "v16.5") {
      name
      tagName
      tagPath
      description
      descriptionHtml
      evidenceSha
      createdAt
      updatedAt
      releasedAt
      assets {
        assetsCount
        sources {
          format
          url
        }
        links {
          id
          name
          url
          external
        }
        evidenceFilePath
      }
      author {
        name
        username
      }
      commit {
        sha
        webUrl
      }
      milestones {
        nodes {
          title
        }
      }
    }
  }
}

Response:

{
  "data": {
    "project": {
      "release": {
        "name": "Version 16.5",
        "tagName": "v16.5",
        "tagPath": "/root/release-test/-/tags/v16.5",
        "description": "This is a small update to version sixteen.\n\nAnd this is an edit!\n\nAnd this is an edit through the API!",
        "descriptionHtml": "<p data-sourcepos=\"1:1-1:42\" dir=\"auto\">This is a small update to version sixteen.</p>&#x000A;<p data-sourcepos=\"3:1-3:20\" dir=\"auto\">And this is an edit!</p>&#x000A;<p data-sourcepos=\"5:1-5:36\" dir=\"auto\">And this is an edit through the API!</p>",
        "evidenceSha": "08f63709250846dc29b9108bcbc90cd3dcecbc9ce02d",
        "createdAt": "2019-11-21T18:45:49Z",
        "updatedAt": "2020-03-11T15:54:44Z",
        "releasedAt": "2020-06-21T18:45:49Z",
        "assets": {
          "assetsCount": 5,
          "sources": [
            {
              "format": "zip",
              "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.zip"
            },
            {
              "format": "tar.gz",
              "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar.gz"
            },
            {
              "format": "tar.bz2",
              "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar.bz2"
            },
            {
              "format": "tar",
              "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar"
            }
          ],
          "links": [
            {
              "id": "gid://gitlab/Releases::Link/7",
              "name": "Blog post",
              "url": "https://google.com",
              "external": true
            }
          ],
          "evidenceFilePath": "http://0.0.0.0:3001/root/release-test/-/releases/v16.5/evidence.json"
        },
        "author": {
          "name": "Nathan Friend",
          "username": "root"
        },
        "commit": {
          "sha": "87fdf474c9e453a8e9ece1353fe7ff915b1e9ccd",
          "webUrl": "http://0.0.0.0:3001/root/release-test/-/commit/87fdf474c9e453a8e9ece1353fe7ff915b1e9ccd"
        },
        "milestones": {
          "nodes": [
            {
              "title": "13.20"
            }
          ]
        }
      }
    }
  }
}

Related to: #208702 (closed), #208724 (closed), !27448 (closed)