* feat: alternate search index for nimbus
Create an alternate search index for Nimbus that filters out
feature-flagged pages (equivalent to setting all feature flags to
false).
Notes:
- Creates two new DB tables, `page_nimbus` and `page_section_nimbus`,
which are filtered versions of `page` and `page_section`
- Makes `nimbus` versions of all the DB search functions
- Refactored the embedding upload script. Changes to make it faster (got
annoyed by how slow it was when testing...), incorporate retries, and
produce better summary logs.
- Upload script, when run with the environment variable
ENABLED_FEATURES_OVERRIDE_DISABLE_ALL, produces and uploads the
alternate search index
- Changed all the search calls in frontend/API to check for
`isFeatureEnabled('search:fullIndex')` to determine whether to search
the full or alternate index
* ci: produce nimbus search indexes on merge
* fix: turn full search index on
