Offer option to turn it on and off per assignment via a checkbox

[xow]

Right now it's automatically enabled for all assignments. That's fine because it won't display any of the structured feedback fields if they are empty. But the convention is to have a checkbox.

"Nice to have" rather than necessary

[tonyjbutler]

Hi John. The plugin actually started out with the standard 'on/off switch' but it just seemed like an unnecessary extra box for the teacher to tick before they could start configuring the actual criteria. So I removed it and made the plugin enable/disable itself automatically based on whether or not any criteria have been defined.

When I get chance I'll add an admin config setting to make the enabling of the plugin a manual checkbox option. I guess there could be some value in being able to define a set of criteria and then temporarily disable the instance without losing the config.