Provide the data export of all data as a flat file

[MartinHinz]

Currently, users cannot download the full database without the API. It is desirable that this be made possible again, and such a possibility will exist again in the update. In order to speed up such a download, it would be good if there were a flat file for it that is created, for example, daily (daily snapshot).

The strategy for this would be to use the existing Ruby structure. In addition, a scheduler gem (suggestion: whenever) should be used to export this flat file (csv) daily.

Then, when a user wants to download all the data, we could offer the daily snapshot, and also use it for the c14bazaar right away.

Tasks:

• install scheduler (whenever?)
• store daily snapshot in a suitable place
• Install download button for daily snapshot

[MartinHinz]

Currently implemented in #338 for json, if you agree, we can follow the same procedure for csv as well?

[joeroe]

@MartinHinz CSV is also implemented, right: https://xronos.ch/data.csv? Can we close this?