Use aria-hidden when rendering SVG avatars

It looks like not all screen readers identify SVG images with empty aria
labels as a decorative image, as reported by the Axe accessibility
engine.

So we're using `aria-hidden` instead, since we don't want the text of
the SVG to be read by screen readers. We're using `aria-hidden` instead
of the `presentation` role for the reasons mentioned in commit
35659d441.

spec/components/shared/avatar_component_spec.rb

@@ -8,7 +8,7 @@ describe Shared::AvatarComponent do
     render_inline component
 
     expect(page).to have_css "svg", count: 1
-    expect(page).to have_css "svg[role='img'][aria-label='']"
+    expect(page).to have_css "svg[role='img'][aria-hidden='true']"
   end
 
   it "shows the initial letter of the name" do