addressed issue https://github.com/globaleaks/GlobaLeaks/issues/300
 temp commit - dummy variables - TODO REBASE: https://github.com/globaleaks/GlobaLeaks/issues/102#issuecomment-23224430
extended fields with preview key for https://github.com/globaleaks/GlobaLeaks/issues/515
updated tip list with preview (handler and test) https://github.com/globaleaks/GlobaLeaks/issues/515
UT aligned to fix to issue 513
moved supported_extensions in GLSetting
implemented date extension and system comment scratch
extended Node and Comment with new fields
updated migration script 5to6 with Node and Comment
Merge branch 'extend_expire_date' of github.com:globaleaks/GLBackend into extend_expire_date