Zoekt: Automatically create zoekt_indexed_namespace records

Problem to solve

Currently, SM customers need to manually add each individual namespace to Search::Zoekt::EnabledNamespace

Proposal

We should:

1. Add a new setting (we can call it index all namespaces in Zoekt)
2. Add a scheduling task that creates Search::Zoekt::EnabledNamespace for all top-level namespaces if the setting is enabled