[META] SVG support OOTB in Backdrop core

[klonos]

• [ ] #5541 | Support SVG in Image Library
• [ ] ???

This topic comes up very frequently in the forum and the Zulip chat. There are currently a few concerns:

• SVGs technically are NOT images - they are XML files. So should they still be allowed as an exception/special-case to be added in the image file type, or not? If not, then which file type is most suitable? ...should a separate type be created specifically for them?
• Since they are not image files, they cannot be handled by image toolkits (no thumbnails or image styles support).
• They can be a security concern: https://blog.online-convert.com/svg-file-and-its-danger

  The danger of an SVG file comes from the fact that it’s an XML that can have embedded CSS and JavaScript. The web browser will automatically run any JavaScript embedded in an SVG file. Therefore, if the script contains malicious code, it will place the user’s computer at risk.

Despite the above, the format is very popular among designers (and browser support is above 99%), so it can be considered a flaw for Backdrop.

[klonos]

Re the security concerns, I have opened #5965, to suggest that we move to using http://htmlpurifier.org for HTML filtering in CKEditor, but I see that there's also https://www.drupal.org/project/svg_sanitizer which is a more purpose-specific solution (uses https://github.com/darylldoyle/svg-sanitizer).