Fix poll answer images not being displayed

After updating foundation-rails in commit 58071fd6, the orbit slider
stopped working properly. That's because the `.orbit-slide` elements now
use a `position: absolute` rule, and so our rule for `.orbit-container`
elements making their height 100% (which we added in order to be able to
add images with different heights) makes them have a height of 0px,
since now the `.orbit-slide` elements are not part of the document flow
anymore.

Making the `.orbit-slide` elements have relative position fixes this
issue, but introduces a different one, producing a really bad-looking
animation when changing a slide.

So we're disabling the animation as well in order to avoid this jump.

This change also fixes another issue introduced in commit 58071fd6 which
caused slide controls to stop working when changing slides back and
forth.

app/assets/stylesheets/participation.scss

@@ -1769,6 +1769,7 @@
 
 .orbit-slide {
   max-height: none !important;
+  position: relative;
 }
 
 .orbit-caption {