BUG Fix sorting on main ReportAdmin grid

Currently trying to sort the reportadmin grid results in a JS error. This is due to the controller routing not properly supporting multiple routes. This fix mocks an 'index' sub-action to expose the main form via ajax.

ref: CWPBUG-133

Depends on https://github.com/silverstripe/silverstripe-framework/pull/3139 and https://github.com/silverstripe/silverstripe-framework/pull/3138