Reworked last step of the mobile tour.
Added method to User instances to update their config. Reworked UserConfigsController to move the code that converts params from string to boolean into a private function.
Move the function that converts string params to booleans to the application.rb file, so that all controllers inherit it.

Use it as much as possible everywhere a string "true" or "false" is passed to a controller in a JSON value and converted to boolean.
UserConfigsController is now able to update both config flags: show_main_tour and show_mobile_tour
When the user closes or completes the mobile tour, it is not shown again in the future.