Allow lag time for task relation type SF

[#1]

Currently Workfront does not allow lag time in a task relation type SF, e.g. I cannot input task relation "4sf-1d", as system prompts an error saying it does not accept negative value. However, this is industry standard that this task relation type must also work with lag time, not only lead time.

[#2]

this is a good idea..! hope we get it soon.