Create new learning nets in the front end, not the back end

[milsen]

Currently, there is a default learning net that is created whenever the learning net of a course is first requested (i.e. when no learning net for the course can be found in the database yet).
This is done by the back end via the switch '"action" : "create"', which results in a call of LearningNet::create() . Since the task of creating this learning net is very simple (much simpler than a recommendation or validation), it could be done by the front end instead. This would require an LGF export in the front end, but that functionality is required by #3 anyway.

Since the current creation of the default learning net already works, it should be decided whether a switch to the front end for this task would have any clear advantages.