Resolve "sub folders of hiddenfolders are not accessible", fixes #464

This PR meant to be a workaround for #464 (closed). How it works

• in CoursewareFolderChooser component, the subfolders of hiddenfolders are not displayed anymore, using new function called validateParentFolder
• using a new mixin called block-folder-validator-mixin, which contains the required functions to validate folders accessibility in component level:

1. validateFolderAccessibility function gets the folder id and loads the folder and checks its parents (recursively)
2. when validation is done, it sets the value of showInvalidFolderMessage.

• on component level using the mixin, validateFolderAccessibility is called in watcher of either folder or file variables.
• in each related component an error messagebox is introduced to display the error when exists!