MDL-36014 cohorts: Allow teachers to view cohorts

defined new capability 'moodle:cohort/viewabove' and property 'visible' for cohorts.
Combining them will allow to make cohorts visible to teachers

AMOS BEGIN
 CPY [visible,core_moodle],[visible,core_cohort]
AMOS END
MDL-36014 cohorts: Support cohort visibility in Upload cohorts
MDL-35618 enrol_manual: Add possibility to manually enrol cohort members