Remove note about exports being usable offline
Refactor ExportsController by changing what the Export object does

Previously it would just be a dump model like object that held onto
techniques, categories and belts. It didn't have any behavior.

I changed that so Export is no longer a model, but instead a service.
This service can then filter the techniques its given to only include
those that are in certain categories and belts.
Rename export service to TechniquesFilter to better reflect what it
actually does
Add comment describing what TechniquesFilter does
Make TechniquesFilter accept an arrays of belts and categories rather
than arrays with misc ids