[BUGFIX] replace non-free jsmin.php with hook

jsmin.php has a non free license. See referenced bug report for
details. This patch replaces jsmin.php with a hook to provide a
way for extensions to implement own compression algorithms.

Change-Id: I1fc5bfe29aaa20692c4323dd28d5c0a95863cf3d
Fixes: #31832
Releases: 6.0
Reviewed-on: http://review.typo3.org/6682
Reviewed-by: Marcus Schwemer
Tested-by: Marcus Schwemer
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn