removing students from a class should also remove them from the section

currently, removing a student from a class only removes them from the global section. removing a student from a class should also cause that student to be removed from all sections they're in.

code reference: https://github.com/haystack/nb/blob/1d33494b101cd981473c389f9231ae235ea648bc/routes/classes.js#L326 https://github.com/haystack/nb/blob/1d33494b101cd981473c389f9231ae235ea648bc/models/utils.js#L142

haystack / nb

removing students from a class should also remove them from the section #141