Fix search feature config API

[fisx]

https://wearezeta.atlassian.net/browse/SQSERVICES-1687

Checklist

• [ ] The PR Title explains the impact of the change.
• [ ] The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• [ ] If this PR changes development workflow or dependencies, they have been A) automated and B) documented under docs/developer/. All efforts have been taken to minimize development setup breakage or slowdown for co-workers.
• [ ] If HTTP endpoint paths have been added or renamed, or feature configs have changed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• [ ] If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• [ ] changelog.d contains the following bits of information (details):
  • [ ] A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • [ ] If new config options introduced: added usage description under docs/reference/config-options.md
  • [ ] If new config options introduced: recommended measures to be taken by on-premise instance operators.
  • [ ] If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.
  • [ ] If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • [ ] If public end-points have been changed or added: does nginz need un upgrade?
  • [ ] If internal end-points have been added or changed: which services have to be deployed in a specific order?