Activity: restore learner commenced and completed activity

Fix: commenced timestamp would not be saved

Fix: next button in course is disabled on last step

Fix: not working umlaut on /group-edit

Fix: the number of steps in a ”question pool” has no effect

Fix: in some cases the commenced, and success values of a learner would not be stored correctly. The code had been adjusted. To fix existing problems, try to "recalculate" the learner.

The nav bar sometimes shows: "Step -1 of 5".