front: input can_backtrack information for the different stdcm endpoint calls

[Erashin]

• when calling pathfinding/blocks to check the path before STDCM (AND ONLY BEFORE STDCM), can_backtrack should be true for every service/driver stop along the path, except for the departure and arrival. The other pathfinding calls (operational-studies) should not allow backtrackings
• when calling stdcm, can_backtrack should be true for every service/driver stop along the path, except for the departure and arrival
• use the new front feature trigger to toggle this behavior. Fallback to using the debug mode as a toggle if more time-efficient