Requeue BBM to backfill namespace_details from projects table

As discussed here #358144 (comment 2803627423), we need to backfill the namespace_details table with the description, description_html, and cached_markdown_version from the projects table where the namespace_details records are missing.

Also refer to #358144 (comment 2807780667) on how to avoid queueing it if it's already enqueued.

Implementation guide

We have already implemented this for the namespaces table in Backfill missing namespace_details from namespaces (!210305 - merged). We need to do the same for the projects table.