[frontend/course administration] Automate task unhiding

[ClemCordier]

Is your feature request related to a problem? Please describe.
Being unable to make a (set of) task visible to the student automatically only after a given (set of) exercise has been successfully answered.
Some task may be related to previous ones and contain (part of) their solution, which may lead students to take advantage of it.

Describe the solution you'd like
Automating task visibility so that a student has no access to some task until he passed other specific ones.

[anthonygego]

Implementation cost should be drastically reduced when #908 is merged as it gathers all accessibility filters in the same place.