FIX: Whitelist marker elements when uploading SVGs (PR #10703)

As <marker> does not seem to have any attributes that affect security. It has been added to the whitelist of allowed elements within an uploaded SVG.

Discussed: https://meta.discourse.org/t/marker-element-being-striped-out-from-uploaded-svg/164432

No tests required for this change as I could see.

GitHub

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/marker-element-being-striped-out-from-uploaded-svg/164432/3

Thanks, this looks good!