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', CFG->disablemycourses is deprecated;
- CFG->frontpageloggedin by default shows list of available courses;
- There is separate item to display course search box
- CFG->maxcoursesincombo is deprecated
- CFG->maxcategorydepth changed default value to 2 since we have AJAX loading now
- FRONTPAGECOURSELIMIT is transformed to CFG->frontpagecourselimit

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