Bug 838224 - "Interim" homepage banners (to support MWC) 

Hi, my first pull request :D 

I've added the same animation to the new elements, but could be changed if necessary.

I have a doubt about how integrate the URLs. I've finally used:
    <a href="{{ _('https://developer.mozilla.org/en-US/docs/apps')}}">
but I'm not sure if this is the correct way... If not, please tell me and I'll correct it.

Un saludo