Cron: Move admin gui from Administration node to own node

[lscharmer]

This PR is part of the following Feature Requests: https://docu.ilias.de/go/wiki/wpage_8602_1357 and https://docu.ilias.de/go/wiki/wpage_8648_1357
This PR moves the administration settings for the Cron component from the Administration to the Cron component itself.
Previously the settings where located at the admin node System Settings and Maintenance. This is moved to a new admin node Cron.
Additionally the following is changed:

• The visible access check is removed as it is stated in the FR.
• All references to the ilCronManager class are replaced with corresponding calls to ilObjCronGUI (linking to the cron tab).
• As the only class left in classes was ilCronManager the folder is deleted.
• As it was a double redirect the method ilObjSystemFolderGUI::jumpToCronJobsObject is removed and calls to it are replaced with direct links to the ilObjCronGUI.

[fneumann]

Found a small issue regarding the position in the admin menu. The rest of the changes in Administration look good to me.

[fneumann]

Thanks for moving the menu entry. I checked out and tested again and found no problems. The code looks good to me.

[mjansenDatabay]

Thx @lscharmer and @fneumann