Create function to preload course contacts for the group of courses

We used to have function get_courses_wmanagers() that loaded course contacts for the list of courses in one big query instead of doing one query per course.

In MDL-36898 the function core_course_renderer::course_info_box() was changed so the course contacts include only enrolled users. But get_courses_wmanagers() was not changed.

In MDL-38147 / MDL-37009 all course contacts are retrieved in one function in class coursecat, at the moment it uses the same code as was changed in MDL-36898 core_course_renderer::course_info_box()

We really need some efficient function/query that loads contacts for the number of courses. They will usually be the courses in the same category but it also can be search results, enrolled courses, available courses on the frontpage that can be from different categories.