Map API Key Validity Check

[dpaulat]

There are often scenarios where users report problems with a black map. Frequently, this is because of an invalid API key, but it is difficult to confirm this as the root cause.

The settings dialog should have Test buttons next to the Map API keys. This will perform a validity check to inform the user whether or not there is an issue with the API key they have entered. The validity check will involve a query to Mapbox and/or MapTiler servers.