Limit maximum number of child epics

What does this MR do and why?

Limits maximum number of child epics to 100. Existing child epics can be still updated even if limit was already reached (we check if parent has changed).

Related to #373700 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Change MAX_CHILDREN_COUNT constant to 1 and restart rails server
2. Try to add an epic to a parent epic which already has some epic assigned

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.