Fix retain cycle and VM recreation

[RuiAAPeres]

What

This PR addresses two issues with the VillagersViewModel. In particular with its recreation whenever SwiftUI dictates so and the implicit use of self as part of its implementation.

How

1. Make self weak and avoid the map's closure capturing self.
2. Use StateObject instead of ObservedObject. Although I am not particular fond of this approach, it fixes the problem where VillagersViewModel is being recreated every time the user switches tab on TodayBirthdaysSection.

This problem can be seen here:

Before	After (with this PR)

We can also see the memory growing with the implicit self capture:

Before	After

[Dimillian]

Perfect!