Merged r11724 from trunk to 2.3-stable.

Add TODO comment about gantt issues sort (#7335).

Sorting by date was dropped by r4581 (#7128).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/2.3-stable@11769 e93f8b46-1217-0410-a6f0-8f06a7374b81