Removed persistence configurability from referrer fields.
Renamed market_source module's human-readable name to Source Code.
Preserving market_source namespace for now.
Updated Market Source Persistence columns to have three options: on,
off, and direct.
Updated market_source.js to respect new on/off/direct options for
source code field persistence.
Merge branch '7.x-4.x-market_source-t335' into 7.x-4.x