Change foldername: exercise/src -> exercise/task

Also adjust some names in files and broken links.

You changed-line spammer!

mentioned in commit 3773ed8f

merged

changed milestone to %2

assigned to @smuething

mentioned in commit ca10c880

mentioned in merge request !22 (merged)

@rhess I get this:

failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial01/exercise/src/src_dir': No such file or directory
failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial06/exercise/src/src_dir': No such file or directory
failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial05/exercise/src/src_dir': No such file or directory
failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial00/exercise/src/src_dir': No such file or directory
failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial03/exercise/src/src_dir': No such file or directory
failed to create symbolic link '/home/dominic/dune/build-opt/dune-pdelab-tutorials/tutorial04/exercise/src/src_dir': No such file or directory

Can you have a look?