Set branch to stable.
Added 2.1-stable branch.
Set version to 2.1.0.
do not show estimated time if it is nil on issue page
add asserting that issue estimated hours is nil when creating minimal issue
gantt: use content_tag instead of html tag at the tooltip
gantt: use content_tag instead of html tag at the label on the right
gantt: remove redundant empty lines from unit gantt helper test
gantt: use content_tag instead of html tag at subject for project
gantt: use content_tag instead of html tag at the markers
