Add a limit on number of child epics per epic
In preparation to allow epics to have children epics from different group hierarchies, we want to add a limit to the number of children can be added to an epic.
The goal of the limit would be to prevent performance degradation in epics with a large number of children (we execute permissions check for each child epic group which can be costly).
The limit value still needs to be determined but initially, we considered 100
as a safe start.