Split get_courses_for_wiki impl

LMS-2914
@sarina To be consistent with the other APIs in this class, I would look at both `get_courses` and `get_course`.  Unfortunately, they are currently not consistent.  `get_courses` takes in a `branch` as a parameter, while `get_course` deduces the `branch` from `self.branch_setting_func`.  My opinion is to make all 3 APIs consistent and modeled after `get_course`.  That is, have them determine the branch based on the branch_setting.