Add API v2

[jfreda]

This PR adds a new v2 of the API, which can be enabled via feature flag. This version of the API uses PostgreSQL data as the source of truth instead of Algolia. One other change of note is that the data consistency checks for Algolia/PostgreSQL now handle slices with elements in differing order.

New feature flag api_v2:

feature_flags {
  // api_v2 enables v2 of the API.
  flag "api_v2" {
    enabled = true
  }
}

[anubhavmishra]

🎉