merge vln/seek
tweak migrations to check columns exist or not before add/remove
change callbacks for  work_groups:  "before_add, before_remove" to "after_add, after_remove" (clean all subscriptions if projects are empty)
set Seek::Config.project_hierarchy_enabled = true in test env.
remove callback to set_default_subscriptions when person is created(it is unnecessary as person has no projects when created)
move sync_delayed_jobs to test_helper
fix test after removing set_default_subscriptions callback before person is created
Merge remote-tracking branch 'lihua/vln-merge' into vln-merge
turn off project_hierarchy for tests