View hierarchy: Do not embed UITabBarController inside UINavigationController

Currently a the `UITabBarController` is embedded inside a `UINavigationController`. It is discourage by Apple and prevent us to customize navigation bar easily for each tab.

Extract from Apple [documentation](https://developer.apple.com/library/archive/documentation/WindowsViews/Conceptual/ViewControllerPGforiOSLegacy/TabBarControllers/TabBarControllers.html#//apple_ref/doc/uid/TP40011381-CH102-SW1):

> Note: Although a navigation controller can be embedded inside a tab, the reverse is not true. Presenting a tab bar interface from within a navigation interface is potentially confusing for users. A navigation interface uses one or more custom view controllers to present an interface focused on one goal, which is usually the management of a specific type of data. By contrast, the tabs of a tab bar interface can reflect completely different purposes in an application and need not be related in any way. In addition, pushing a tab bar controller on a navigation stack would cause the tabs to be displayed for that screen only and not for any others.

Below current view hierachy: