edit: classroom dataclass

[faretek1]

• change site.classroom.Classroom into a dataclass
• fixes a minor bug when connecting to mystuff_classes (due to changes when decoding session id)
• uses commons.headers instead of importing headers (just reduce number of global vars)
• fixes a bug where all classroom activity raw strings return the default case of "{user} performed an action."

[faretek1]

It is practically impossible to currently completely exhaust all possible bugs. But if we are to implement automated testing, and people report bugs, any things I missed would be solvable. In addition, most people do not use the classroom class so I will be able to test this myself

I do not see any obvious reason why this pr would break things right now, as the basic functionality works on my device