Add refresh functionality

[IoanaAlexandru]

Information in the app is generally cached, so if something is updated in the db you usually need to restart the app for the changes to become visible. It would be nice if pages had a refresh animation (drag from the top to refresh - clear the provider cache and fetch the data again). This is not high priority though, as data doesn't change that often and there are other methods to refresh (e.g. changing the filter for the timetable/websites).