Only generate global whitelists on demand

[egli]

Right now the global whitelists are generated by a cron-like job every night.

Instead we should run the nightly job only if there have been any changes to the global words.

The implementation would be fairly similar to the way it is done for the local tables. Have a dropping-chan listening to changes in the global tables. Once we have an event then wait for a trigger on the clocking-chan which fires once a night. Once it fires write the whitelists.

With this implementation it would still run it twice (as the buffer is of size 1, while we wait for the clocking-chan) so it is not clear if this added complexity is worth it.