Assignments+groups: Allow one single assignment to be assigned to all groups

[ywarnier]

Current behavior / Resultado actual / Résultat actuel

You can only create group assignments by creating one assignment in each group.

Expected behavior / Resultado esperado / Résultat attendu

It should be possible to create an assignment from the normal assignment tool and check an option that says: "This is an assignment by groups", and the assignment would get created equally for all groups.
This only requires a database change in the c_student_publication table because otherwise we can't store the fact that it is a common assignment.

Chamilo Version / Versión de Chamilo / Version de Chamilo

1.11.2

@p4pr8n for more info