Internal & External images added through overlay.

Internal or External images can be easily added through overlay.
Modified "insert_picture" function which return all internal images with grid_fs_obj.filename.
 
Please test the following after taking this pull:
1. Go to edit/create view of any resource or page where orgitdown editor is available.
2. Click on "Picture" button.
3. Overlay with two radio buttons is available
4. if External :
    >>  which ask for "Image Description", "Width" (image width in pixel) & "URL" (image absolute url).
5. if Internal:
    >> which ask for "Image Description", "Width" (image width in pixel) & all internal images will get visible on the drop down box under URL label.
 6. Add Image  
 7. Above image will get added in orgitdown editor (org_mode format )
 8. Save