Use GitLab SVG icons in file_type_icon_class helper

file_type_icon_class returns an icon name based on the file type provided in params (folder, file pdf, file image...). Here is the list of icons and mapping to GitLab SVGs:

• folder -> folder-o
• archive -> archive
• share -> share (doesn't seem to be used)
• file-pdf-o -> document
• file-image-o -> doc-image
• file-archive-o -> `doc-compressed`
• file-audio-o -> volume-up
• file-video-o -> live-preview
• file-word-o -> doc-text
• file-excel-o -> document
• file-powerpoint-o -> `doc-chart`
• file-text-o -> doc-text

There are currently 2 helpers that use file_type_icon_class:

• tree_icon
• blob_icon