Styling changes for task group reordering
Frontend part of task group ordering
Server side part for task group sorting
Set sort_index for all existing task_groups
Merge remote-tracking branch 'origin/develop' into feature/workflow-task-group-sorting

Conflicts:
	src/ggrc_workflows/assets/javascripts/workflow_mustache_helpers.js
Add blank string defaults for `sort_index` columns
Avoid script error when sorting result list
Merge pull request #1585 from reciprocity/feature/workflow-task-group-sorting

Task group sorting