MDL-61215 core_files: add new optimised_image group

Currently, PHP getimagesize method doesn't support SVG images.
As some features, such as badges, processs and optimise the images
before using them, a new filetype group has been created to exclude
SVG from there: optimised_image.
SVG can't be removed from web_image because then users won't be
able to add SVG images to their courses using labels, pages...

lib/filelib.php

@@ -1639,6 +1639,7 @@ function download_file_content($url, $headers=null, $postdata=null, $fullrespons
  *     commonly used in moodle the following groups:
  *       - web_image - image that can be included as <img> in HTML
  *       - image - image that we can parse using GD to find it's dimensions, also used for portfolio format
+ *       - optimised_image - image that will be processed and optimised
  *       - video - file that can be imported as video in text editor
  *       - audio - file that can be imported as audio in text editor
  *       - archive - we can extract files from this archive