External sessions are deleted after they have been used
Merge branch 'develop' of https://github.com/open-steam/webarena into develop

# By Felix Winkelnkemper (7) and Dominik Lüke (3)
# Via Dominik Lüke
* 'develop' of https://github.com/open-steam/webarena:
  bugfix regarding cut'n'paste
  bugfix regarding cut'n'paste
  cut'n'paste and copy'n'paste for paint
  - switched from enclosements to intersections - improved repositioning
  Added a simple group object
  Removed old psitioning logic Implemented new positioning logic TODO: Care for mustnot areas
  Added hiding support for IconObjects
  lines and arrows now can be dashed or dotted
  The server now has a debugMode. When this is turned on  - the Webserver starts with an overview page  - the server does not catch uncatched errors resulting in crashes when errors occur  - the client code contains line numbers
  Corrected an error where objects could not be moved when they had malformed link attributes