Improvements cron plugin

[PLanB2008]

Using the cron plugin should be improved.

There are 2 main improvements we should tackle:

• holiday.yml is not always readable by the global-admin / school admin.
• linuxmuster / sophomorix commands need to be run via sudo

For a user its not easy to understand why a cronjob is not running like intended.

I think first of all the webui should create the holiday.yml with read access for all admins. The second issue is not as straight forward, but I think we could improve this.

[kiarn]

There's now the possibility to switch from globaladministrator to root when editing crontab.