Add an option to use the cached dataset for longer

[bopjesvla]

Leon writes in #103:

On the road, the mobile data will get drained and waste a lot of bandwidth. Every ten minutes, the cache gets invalidated as long as there changes (right?). It will reload if there is an internet connection, requiring you to download all the data again.

This is indeed the case, but syncing the dataset seems too complex a solution for this. It's easier to let the user choose how long the cached version is valid, probably in the main menu. This requires some communication between the app and the service worker.

[leon-wbr]

I support this. It's a question of UX/UI, also.

Perhaps we could implement an endpoint that returns a datetime of last update and do a notification on initial page load that prompts the user to update manually, if they haven't updated since x amount of time and there are changes?

Bit simpler than implementing a control. Perhaps we could also put the control into that notification to do the initial setting; such as: "There is a new dataset available. Do you want to update now? Yes / No / Automatically update if older than 10 mins, 1 hr, 1 day, 1 week, 1 month."