Use Postgres built-in search functionalities instead of Elasticsearch?

Postgres has been improving full text search and trigram search a lot since version 9.6. I wonder whether it is possible to use them to replace Elasticsearch. If we can, the backend architecture (and deployment) can be greatly simplified. With the `GIN` or `GIST` indexes on search fields, we don't have to worry about the index updates (which invoke `celery` jobs right now).

Right now, Elasticsearch is being used to search genes and genesets. We have **312,983 genes** and **408,237 genesets** in Postgres backend database.

No files, tests, or entry points are named. Start by locating the current Elasticsearch search implementation and Celery index-update jobs, then assess the existing gene and geneset search behavior against the proposed PostgreSQL indexes. Done would require a concrete feasibility decision and an agreed migration scope.

Written by the indexing model from the issue text.