Extension registry GraphQL API is slow

Created by: felixfbecker

Currently, the extensions registry page usually takes a long time to load, sometimes even up to minute, because of the GraphQL request to query extensions.

Worse, this request is not only done on the registry page, but also when loading extensions.

And since private instances query extensions from sourcegraph.com, this experience is true for all private instances too.

The primary task here is a backend issue. It's likely that our DB tables and queries are not optimized (e.g. could it be that we have extensions and extension releases in the same table and run a SELECT DISTINCT on every query?).

Related: #1983 (closed)

cc @sourcegraph/core-services