add a site setting for allowing animated avatars

:sparkles: This adds the `allow_animated_avatars` site setting (defaults to `false`). :sparkles:

![nod](http://media.giphy.com/media/4FvmJftU7yreg/giphy.gif)

#### Presentation

When a user uploads a gif for her avatar, it will use only the first frame when animated avatars are disabled.

#### Nice touch

Because avatars are only squares, we might crop the uploaded image when generating the thumbnails on the server.
While we're waiting for the server to finish this background task, we return to the client the url of the uploaded file. We'll simulate that eventual croping on the client-side to prevent the image from being stretched by the browser.

#### Note

:warning: This is not retroactive. If one of your users already uses an animated gif, you will have to upload it again to make it unanimated. I'll add a rake task later to make it easier though.