Add #integrationGitlabUser1 for easy GitLab integration testing

Add #integrationGitlabUser1 for easy GitLab integration testing

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2393

Split off from https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702

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Usage: (you can see actual usage of it in https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702)

fixtureLoader.ensureIntegrationEnvironment('#integrationGitlabUser1');

  const fixture = fixtureLoader.setup({
    userGitlab1: '#integrationGitlabUser1',
    user2: {}
  });