Mobile friendly views for alerts

Closes #663 (closed)

The following has been changed:

• small alert cards have now a new two rows layout (used for small screens only)
• in list views the alerts are displayed now in a grid that takes up the full screen width
  • the grid column count depends on screen width (mobile has just one column)
  • this was discussed with Kirsi but sketches have not been updated to reflect it
• the header containing links has now a mobile version that collapses the contents into two smaller menus

A note for tester: weather-notification service has a script npm run forecast-handler that can be used for testing to run locally created alerts.