[boilerplatetutorial] Remove bitrot and clean up

- Make sure to include config.h at the start of all main
  programs. config.h must be included before any other headers and the
  include statement should really be in the main .cc file itself and not
  in some header like pdelab.hh.

- leafView() -> leafGridView()

- Fix some warnings about unused typedefs / variables

- Remove code setting UG parameters from programs that do not use UG

- Explicitly set correct mass matrix integration order in poisson_adaptive
[Poisson] Adapt to recent changes in StationaryLinearProblemSolver

- use non-deprecated constructor
- explicitly turn on hanging nodes handling if required