addressed issue: https://github.com/globaleaks/GlobaLeaks/issues/112
added experimental hardcoded limit to avoid mail-floods https://github.com/globaleaks/GlobaLeaks/issues/552
implemented date extension and system comment scratch
extended Node and Comment with new fields
updated migration script 5to6 with Node and Comment
fixed format not converted since 5 to 6: https://github.com/globaleaks/GlobaLeaks/issues/515