Progress bar for students is not displayed on a fresh repository

[yurug]

If a fresh empty /sync is used, the progress bars for students is not displayed.

@nobrakal suggests that this is due to the absence of exercises.json. This file can be created by changing the state of exercises from open to close, and to open again. Once this operation is done, the progress bars appear.

[erikmd]

Just FTR, this is related to this other open issue:

#337 (comment)