Enable indexed search by default on sourcegraph/server

Created by: sqs

Indexed search is currently not enabled by default for sourcegraph/server because it has unbounded resource requirements that @keegancsmith (rightly) deemed likely to overwhelm single-node instances. It is enabled by default on local dev and on cluster deployments. See https://sourcegraph.slack.com/archives/C07KZF47K/p1549416905712500?thread_ts=1549416905.712500 for context.

It would be good to enable it by default on sourcegraph/server (and do the requisite work necessary to make that not overwhelm a server).

Related to https://github.com/sourcegraph/sourcegraph/issues/2165 (that is about docs).

cc @slimsag

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