Admin page hangs when OC server is down

[tgloeggl]

• Cause are the two functions, which try to directly interace with OC during retrieval of configurations
• The lists, theses functions provide need to be moved to a separate table und be updated by a cronjob

The functions: https://github.com/elan-ev/studip-opencast-plugin/blob/master/lib/Models/ScheduleHelper.php#L141 https://github.com/elan-ev/studip-opencast-plugin/blob/master/lib/Models/ScheduleHelper.php#L170

[tgloeggl]

see also: #551