Offer Opportunity to interrupt a trigger when the config is false

[NinaHerrmann]

Hitherto, triggers are able to throw exception. However, when the config settings for a trigger are wrong this will lead to a repeatedly useless call of the trigger for every course.
Possible solutions are:

• providing a method which allows triggers to abort the trigger,

• implementing a check_validation() function in the core which checks a trigger before it is called and disables it in case the config settings are wrong.

[NinaHerrmann]

learnweb/moodle-lifecycletrigger_byrole#5