When avatar/header are GIF, generate static versions (#1428)

* When avatar/header are GIF, generate static versions.
Account API returns "avatar"/"avatar_static", "header"/"header_static"
Static version is the same as original for other cases
Web UI de-animates avatars in toots, lists of users

Fix #441, fix #596, prerequisite for #1064

* Fix JS test

* Add rake task to generate static avatars/headers from GIF ones, add test

app/assets/javascripts/components/components/status.jsx

@@ -90,7 +90,7 @@ const Status = React.createClass({
 
           <a onClick={this.handleAccountClick.bind(this, status.getIn(['account', 'id']))} href={status.getIn(['account', 'url'])} className='status__display-name' style={{ display: 'block', maxWidth: '100%', paddingRight: '25px' }}>
             <div className='status__avatar' style={{ position: 'absolute', left: '10px', top: '10px', width: '48px', height: '48px' }}>
-              <Avatar src={status.getIn(['account', 'avatar'])} size={48} />
+              <Avatar src={status.getIn(['account', 'avatar'])} staticSrc={status.getIn(['account', 'avatar_static'])} size={48} />
             </div>
 
             <DisplayName account={status.get('account')} />