Clarify disable submissions option in assessment handin settings

[jhs-panda]

This is an issue raised by 213 head TA. For labs such as bomblab and attacklab, where the intended submission format is through the terminal (not on Autolab), sometimes students submit to Autolab through web and automatically get assigned 0s. The only current solution is to reset everything and clear cache for the student, but it still loses the bomb explosion information.

Proposed solution is to disable Autolab submissions on website at assessment level.

Existing solution exists (15213 does not know): Disable handins when Edit Assessment > Handin: Resulting look: The meaning of disable handins is a bit confusing, so can make the message more understandable to users/option more known.