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/account.jsx

@@ -65,7 +65,7 @@ const Account = React.createClass({
       <div className='account'>
         <div style={{ display: 'flex' }}>
           <Permalink key={account.get('id')} className='account__display-name' href={account.get('url')} to={`/accounts/${account.get('id')}`}>
-            <div style={{ float: 'left', marginLeft: '12px', marginRight: '10px' }}><Avatar src={account.get('avatar')} size={36} /></div>
+            <div style={{ float: 'left', marginLeft: '12px', marginRight: '10px' }}><Avatar src={account.get('avatar')} staticSrc={status.getIn(['account', 'avatar_static'])} size={36} /></div>
             <DisplayName account={account} />
           </Permalink>