Filepicker, Server files, Improve tree view (Useability)

1. Login as admin, create Folder resource, access filepicker from both Add Files and TinyMCE insert image and video. 2. Observe the directory structure in Server Files. Make sure that navigation is easy, folders are not empty and number of subfolders is not too big 3. Login as teacher and do the same. Make sure you can see only courses where you are a teacher, that you don't see course categories Note: if you login as a student, the Server Files repository will be empty. There is a separate issue MDL-32058 to hide repository from students. Also repository may appear empty if there are no available files with the accepted filetypes