Extend allowed tags and attributes (svg)

[tibvdm]

This pull request adds additional SVG tags:
<defs> <use>

And attributes:
color

• Tests were added
• Documentation update can be found at dodona-edu/dodona-edu.github.io#

[bmesuere]

Is this really needed? There is a reason we filter svg's and use is one of the tricky elements which can be exploited.

[pdawyndt]

@bmesuere We have an assignment in today's exam, where the goal is to generate an SVG that makes use of the use-tag. The SVG contains repeated patterns, and the use-tag is used to keep the generated output as small as possible. When the output grows too large, Dodona no longer diffs the output and it becomes really hard to see for the students where some (possible small) differences are located.

[bmesuere]

I that case is propose to allow it for now, but disable it again after grading is done.