Pull to refresh doesn't work when there are no tasks

Steps to repro:

- Load app, delete any existing tasks so that "You have no tasks!" is displayed
- swipe vertically downwards

Expected outcome

- The loading spinner is displayed

Actual outcome

- Nothing happens

Further details:

Pull to refresh is implemented in the `LoadingContent` composable. This either displays empty content **or** if it is non empty (specified by the logic `tasks.isEmpty() && !loading`) a container which supports pull to refresh.

So the problem is that the empty content should be displayed inside a container which supports pull to refresh.

Additionally the code is currently using `SwipeRefresh` from the accompanist library which is deprecated and needs migrating https://google.github.io/accompanist/swiperefresh/#migration