Modify dashboard group classes endpoint to return class data

[Carifio24]

This PR updates the /dashboard-group-classes endpoint introduced in #93 to return the data for each class, rather than just its ID.

The new format of a successful request will be

{
    classes: [<class_1_data>, <class_2_data>]
}

where each class data object will have the id and name, etc.

CC @johnarban