Remove alt and title attributes from trail images

I removed alt and title attributes from images in trails.

### Why
- this information do not bring any value to our readers (no hovering of image elements on mobile)
- It's actually a problem for visually impaired users who get extraneous noise-like information through assistive technology.
- More data to download (~4KB on the homepage at this instant)

### Accessibility

Also, I added an aria role (role="presentation") to tell assistive technologies not to take the link into account. The title of the story is the only relevant thing here, no need to have 2 links per story for a user using a screen reader.

![Plouf](http://i.imgur.com/bF5Euki.gif)