USSD: Allow entering coordinates instead of using geolocation

[tuxpiper]

Add configurable flag on the USSD engine (environment variable at this point).

That enables offering users to enter a specific mode for submitting coordinates instead of geocoding

1. Flow reaches a location field
2. The question sent to the user asks:

Please enter a search term for location.
Type [%] in order to submit coordinates instead

Aha! Link: https://ushahiditeam.aha.io/features/PROD-92