Use React Navigation

Goal is to replace react-native-router-flux with react-navigation, which is the most supported navigation library now.

Upgrading to RN 0.57 forces us to upgrade also react-native-router-flux as the v3 we use is not working with RN 0.57 and is not supported anymore.
v4 is not compatible with v3 and is based on react-navigation, so while upgrading to v4 we might as well try to use directly react-navigation and remove react-native-router-flux dependency.

https://reactnavigation.org/

For the first POC:

• Loading user credentials from Async storage;
• Handling authentication if user not authenticated
• Displaying the Search screen with the ability to do a search
• Navigating to folder and viewing document
• Not interested in actions on a document/folder (in the top bar)