Removed "Back" button from all tours.

It was too much trouble. Because of the dynamic nature of the app pages, elements could move or disappear which could cause tour bubbles to be misplaced or missing when going back. It's simpler to just remove the Back button. Tours can be reset from the profile page if the user wants to go through them again, anyway.
Mobile tour step now points to a button-group wrapping the show/hide sidebar buttons.

This way the tour points to an always-visible element, which fixes positioning problems.
Mobile tour step is now targeted to a navbar element, with an offset.

This ensures that the targeted element is always visible, which fixes some positioning problems. The offset is intended to make the bubble arrow point at the swipeable area, instead of pointing to the navbar (which cannot be swiped).