Allow using custom endpoints for Geosubmit API

[mjaakko]

Currently the application is hardcoded to send data only to Mozilla Location Services, which is going to shut down soon and does not accept data submissions anymore. Allow the user to specify a custom endpoint where to send data. Note that the endpoint might require an API key - there should be a way to set the API key in that case

Optionally allow using multiple endpoints

This feature is also needed to avoid showing NonFreeNet anti-feature flag in the F-Droid listing