End-to-end encryption

[JohnPalmer]

Implement a system for end-to-end encryption of data being sent to survey APIs, similar to the one used in original Space Mapper.

[JohnPalmer]

This is now almost complete using fast_rsa. We now just need to deal with project-specific public keys and turn the feature on. (It is off for testing so that it is easier to see the data as it comes in. See lib/models/send_data_to_api.dart 13, 40, 43, 50.