iOS: use the standard back button

[tadamcz]

Describe your feature request On iOS the back button (to go back to the list of notes from a note), is custom. It is not the standard back button from Apple.

I think this custom button is worse than the standard button, because the "tappable" surface area is smaller. For example, tapping the very left edge of the screen on the navigation bar does not go back in FSNotes, but it does go back in all apps that use the standard button.

Apple guidelines recommend using the standard:

Use the standard back button. People know that the standard back button lets them retrace their steps through a hierarchy of information.