Implement fallback which geojson file is loaded when date of dashboard not available for a source

Thinking this through again, @reisecker. Scenario... We want stations to show up in the dashboard though their simulations broke down. I think this would really just be for the worst-case scenario that the toolchain failed completely and QMAH was not running for the last runs. If QMAH runs then single stations would still show up now though their simulations started to fail, no? As long as the pro-file or maybe even just the input SMET form when it was working is still there this should work.

Noting that generally the timestamps of all toolchains should align, so that calculating the distributions for the time series is consistent for each timestamp. If timestamps would vary between toolchains then different data would be used for different timestamps.

Another note: How long do we want to allow for this fallback if we implement one? 1 day/week?