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/features/compose/components/navigation_bar.jsx

@@ -17,7 +17,7 @@ const NavigationBar = React.createClass({
   render () {
     return (
       <div className='navigation-bar'>
-        <Permalink href={this.props.account.get('url')} to={`/accounts/${this.props.account.get('id')}`} style={{ textDecoration: 'none' }}><Avatar src={this.props.account.get('avatar')} size={40} /></Permalink>
+        <Permalink href={this.props.account.get('url')} to={`/accounts/${this.props.account.get('id')}`} style={{ textDecoration: 'none' }}><Avatar src={this.props.account.get('avatar')} animate size={40} /></Permalink>
 
         <div style={{ flex: '1 1 auto', marginLeft: '8px' }}>
           <strong style={{ fontWeight: '500', display: 'block' }}>{this.props.account.get('acct')}</strong>