Allow instant redirect to the referenced course

Based on the idea by Matt Gibson who did this in his fork. This does not affect
users with the permission to fetch grades manually so they do not loose this
option.
Describe recent changes in the README file
Fix missing capability names
Check for the permission before fetching grades on activity update

In a rare case when the teacher does not have permission to fetch grades
manually, do not fetch grades on the subcourse activity update.
Adding event observers
Removing manual grades fetching
Fixing fetch_grades task schedule to run once a week (with event observers it's not needed anymore, leaving it just to be shure)
Removing instantredirect field
Fixing missing subcourse:addinstance string
Bumping version.php