Fixed loggedInSelector and moved selectors to separate file

Closes #74

Previous behaviour

loggedInSelector still used the navigation reducer, which no longer exists since we switched to React Navigation. Because of this, the deeplinking saga crashed and caused the app to get stuck at the splash screen.

Steps to reproduce:

1. Open a deeplink, like https://thalia.nu/events/1/.
2. Notice the app is stuck on the splash screen.

New behaviour

The loggedInSelector is updated to use the new status field in the session state. In order to avoid a circular import, the selectors have been moved to a new folder, which will also contain any future selectors we add.

Steps to validate that it works:

1. Open a deeplink, like https://thalia.nu/events/1/.
2. Notice the app handles it correctly.