[WIP] Sponsors page

Basic implementation of #4. The marquee was already made, but I wrote the behat scenario for it.
What's still left to do:
 - [x] Sponsor listing page
 - [x] Sponsor detail page
 - [ ] Styling for both pages
 - [ ] Better implementation of the sponsor slugs (perhaps by creating a more general translation or wrapper)
 - [ ] Behat tests for the pages

What I don't know yet, is how to get to the sponsors listing from the homepage. For now, I made the sponsors header a link, but perhaps we should also make links to the detail page from the sponsors images in the marquee.

Also, if anybody has ideas for the styling and/or the implementation of the sponsor slugs (mostly, how to separate the functionality), please let me know :sweat_smile: