Make this app hub-agnostic

[Bisaloo]

• Remove all hardcoded links
• Use information from config file
• Compare to https://github.com/KITmetricslab/covid19-forecast-hub-de/tree/master/app_forecasts_de to find what should be made more general

[Bisaloo]

The biggest challenge at the moment is figuring out how to pass truth data without hardcoding it. Passing it as a parameter like the forecast data is theoretically possible but it might(???) make the app even slower.

[Bisaloo]

On the other hand, it's not realistic to be able to use our instance hosted at shinyapps.io for all cases. A more reasonable model would be to deploy the app for each hub with their parameters.