MDL-38592 Settings for courses lists on front page are made more user friendly

- 'List of courses' is split into 'List of courses' (available) and 'Enrolled courses (legacy)', ->disablemycourses is deprecated;
- There is separate item to display course search box
- ->maxcoursesincombo is deprecated
- ->maxcategorydepth changed default value to 2 since we have AJAX loading now
- FRONTPAGECOURSELIMIT is transformed to ->frontpagecourselimit