Working with external references in filemanager

Ensure that MDL-28666 and MDL-31901 work together.

What we need to make sure:

1. Files that are references to external resources (let's call them REF) must have special shortcut icon in filemanager

2. When user selects a REF file in filemanager, he can see the 'Origin' of the file in the way that original repository wants to show it.

3. (wish list) Repository return different detail level of original location for the user who is owner of the REF file and anybody else. Trick is that the owner is not definitely the user who is currently editing.

4. Files in 'Private files' or 'Course legacy files' that are themselves the source of references of other files in the system (let's call them SRC) need to have "link" icon in filemanager. When user tries to remove/rename/overwrite SRC file he is warned that all ## existing references will be updated/converted to copies.

5. When SRC file is updated during editing (i.e. author/license information is changed, or file was overwritten by another file with the same name), all references to this file remain!

6. When SRC file was deleted during editing (or moved/renamed which resulted in that there is no file in the original location), all references to this file must be converted to copies.