RFC 624: (M5) Make recognizer behavior configurable

Created by: efritz

Implements a portion of RFC 624: Configurable auto-indexing recognizers.

After deciding on a target API design in #33040:

• Create a new set of Lua builtins (the "sg" script available inside the sandbox; will likely be mix of Lua + Go code)
• Update our default recognizer implementations to use new API
• Add a database table, store methods, and GraphQL resolvers to expose optional recognizer script configuration
• Create a quick-and-dirty site admin page where the recognizer script can be updated
• Inform design we've created a mess