Qualify emperor's children

BUR #34773 has been rejected.

rename emperor's_children -> emperor's_children_(warhammer)

Way too ambiguous.

Also make them a subtag (IDK the term) of CSM like some of the other traitor legions are.

The bulk update request #34773 (forum #323586) has been rejected by @evazion.

It doesn't conflict with anything and other Adeptus Astartes chapters don't have qualifiers unless they conflict with something.