Allow course element folder to access directories from course folder

The course element folder is used to distribute files or to collect files from the learners. In the first case it is often the case that the files are already available in the course folder and thus there is a desire to reuse those files.

Currently the course element folder does always store its files outside the course folder. There is no way to publish data from the course folder or from the attached resource folder.

The goal of this issue is to make this configurable.

• Create new config controller with a selection: automatic folder (as today) or from course folder (new)
• Default is automatic
• Update node config defaults from existing nodes.
  • targets: courseelementdata, sharedfolder, coursefolder
  • subpath for sharedfolder and coursefolder
• If selected the new option, the node is marked as "with errors"
• Two buttons are added when the new option is selected: "Choose folder" and "Create folder"
• Choose folder brings a dialog with the directory structure of the course folder
• The folder chooser must included the resource folder with configured
• Create folder brings an dialog with an input field to create a path
• The path must be checked against existing directories
• The path must allow only save characters
• In the runtime, the new option is checked
• When a course folder is configured that does not exist, the path is created on the fly with
  • one exception: if the folder path points to the resource folder and there is no resource folder configure anymore (target= sharedfolder). In that case an error should be displayed.
• When mounting the course via WebDAV it should behave as expected: content appears under the respective virtual folder
  • MergedCourseContainer.addFolderBuildingBlocks()