Partially revert d9cb907c: "Avoid loading objects from DB in ES results"

What does this MR do?

Reverts some load-from-elasticsearch code that is causing issues on production, introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12691

With these changes, we go back to loading records from the database, while keeping many unrelated improvements that can be found in the MR, like pushing down the pagination into elasticsearch.

By itself, this would reduce performance somewhat, so I'm also going to investigate adding some eager-loading code at the same time to address database N+1 issues.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #12091 (closed)