message: simplify the default authorization query case
  When we added user correlation support, we broke any searchers (like the one in github-auth) that are expecting to only ever support `get` as opposed to `search`. `get` is simpler and often more efficient, so we should use it explicitly when there's no custom user correlation query.
diff: @@ -191,29 +191,22 @@ class Authentication {
  if (!user.data || !user.data.type) { return; }
  let have;
  - let query = source['userCorrelationQuery'] || (
  - user.data.id ? `{
  - "filter": {
  - "type": "{{data.type}}",
  - "id": { "exact": "{{data.id}}" }
  - },
  - "page": { "size": 1 }
  - }` : null);
  -
  - if (query) {
  try {
  + let query = source['userCorrelationQuery'];
  + if (query) {
  let compiled = Handlebars.compile(query);
  - have = await this.userSearcher.search(JSON.parse(compiled(user)));
  + let searchResult = await this.userSearcher.search(JSON.parse(compiled(user)));
  + if (searchResult.data.length > 0) {
  + have = { data: searchResult.data[0] };
  + }
  + } else {
  + have = await this.userSearcher.get(user.data.type, user.data.id);
  + }
  } catch (err) {
  if (err.status !== 404) {
  throw err;
  }
  }
  - }
  -
  - if (have && have.data && Array.isArray(have.data)) {
  - have = have.data.length ? { data: have.data[0] } : null;
  - }
  let madeIndexUpdate = false;
  if (!have && source.mayCreateUser) {
  