Split off site specific code from fsainsert.bg into individual scripts

[Rohaq]

No-one likes a monolith. If we're want to support more sites, it's worth fixing this early.

• Code specific to scraping and inserting into each site should be split off into their own js files, and explicitly specified in the manifest.json for each site individually.
• fsainsert.js should be updated to accept input from any of these scripts, and used to handle messaging to fsabg.js, returning required FSA data to requestor. It might also be worth renaming to something sensible.