Add WAI-ARIA attributes to graph elements

This commit adds role=presentation and aria-hidden=true to all
SVG elements and figcaptions used for linelabels.

This prevents screenreaders from reading out the elements.

As noted in the comment, the WAI-ARIA spec says we may do this
with caution to improve the experience for assistive technologies.
Merge pull request #138 from alphagov/61980510-hide-graphs-from-screenreaders

Add WAI-ARIA attributes to graph elements