Limit number of active submissions from a user

[mpartel]

Accept max 10(?) submissions in the processing state per user.

[jamo]

Make the setting configurable per course, but this is important feature as the usage increases

[mpartel]

Also, one could play with the priority field so that a user that has made lots of submissions recently gets a lower priority. But a limit is needed nonetheless.

[jaapo]

I'll start working on this