change final Submission

• In this MR the latest sumbission logic is changed
• Previously: simply the latest submission was chosen, which resulted in the issue that you could not unsubmit
• Now: There is a boolean added which denotes the latest submission, when the user wishes to unsubmit thee boolean of the latest submission will be set to false (this cal also be viewed by the user)
• Essentially all logic stayed the same, only the latestsubmission function changed and when posting a new submission the latest one (if there is one) will be set to false
• Changes naming from latest to final
• adds transaction to make sure no concurrent calls are made
• enables teachers to make submissions on behalf of a group
• Migration which marks the final submissions properly

fixes #309 (closed) #126 (closed)