Add basic Release GraphQL endpoint

What does this MR do?

Adds a basic subset of the data currently available in our REST Release API to our GraphQL endpoint.

What doesn't this MR do?

This MR does not implement the assets field (which is currently available through the REST Release API). This property was left out in an effort to make this MR smaller and easier to review. This field will be added in a follow-up MR.

Feature flag

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

Example query/response

Getting a single Release by tag name

Click to expand query

{
  project(fullPath: "root/release-test") {
    release(tagName: "v16.5") {
      name
      tagName
      tagPath
      description
      descriptionHtml
      evidenceSha
      createdAt
      updatedAt
      releasedAt
      author {
        name
      }
      commit {
        sha
      }
      milestones {
        nodes {
          title
        }
      }
    }
  }
}

Click to expand response

{
  "data": {
    "project": {
      "release": {
        "name": "v16.5",
        "tagName": "v16.5",
        "tagPath": "/root/release-test/-/tags/v16.5",
        "description": "Release notes here.",
        "descriptionHtml": "<p data-sourcepos=\"1:1-1:19\" dir=\"auto\">Release notes here.</p>",
        "evidenceSha": "08f63709250846dc29b9108bcbc90cd3dcecbc9ce02d",
        "createdAt": "2019-11-21T18:45:49Z",
        "updatedAt": "2020-04-09T14:50:42Z",
        "releasedAt": "2020-06-21T18:45:49Z",
        "author": {
          "name": "Nathan Friend"
        },
        "commit": {
          "sha": "87fdf474c9e453a8e9ece1353fe7ff915b1e9ccd"
        },
        "milestones": {
          "nodes": [
            {
              "title": "13.20"
            }
          ]
        }
      }
    }
  }
}

Getting a list of Releases

Click to expand query

{
  project(fullPath: "root/release-test") {
    releases(first: 5) {
      nodes {
        name
      }
    }
  }
}

Click to expand response

{
  "data": {
    "project": {
      "releases": {
        "nodes": [
          {
            "name": "v16.5"
          },
          {
            "name": "Version 16.4 (pre)"
          },
          {
            "name": "Version 16.3"
          },
          {
            "name": "Version 16.2"
          },
          {
            "name": "An API Release"
          }
        ]
      }
    }
  }
}

Related to: #208702 (closed), #208724 (closed), !27448 (closed), !27614 (closed)