Revisit updates forecast and alerts

[vegardb]

How often and how to update forecasts and alerts.

• when app gets focus?
• some time between updates?
• cahce headers?
• if-modified-since?

[havardf]

My current idea here is to update alerts and forecast every time the following happens:

• The user switches back to the app on his phone.
• The user navigates back to the main screen.

Seems like this needs to be solved in one of two ways:

• running a background job
• Using a combination of 1. a hook using AppState (for catching focusing back the app) and 2. useFocusEffect (to handle navigating back to the main screen.

I am not sure about the consequences of running jobs in the background on people phones, so I would prefer to start with the option of AppState and useFocusEffect.

Example of how to use AppState in this issue: https://github.com/react-navigation/react-navigation/discussions/11851