Notification of when a tile is marked done. helps validation

[johnwhelan]

Currently I check four projects each day for done tiles, three are moving slowly so if I could be notified when a tile had been marked done that would save me having to open the project. Saves processing time on the tile server since I wouldn't need to open each project to check for progress.

Saves by time as well.

Thanks John

[aawiseman]

Something similar was mentioned in #889 -- a notification after a certain number of edits, "hey, this task needs validating"