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/stylesheets/components.scss

@@ -1,7 +1,7 @@
 @import 'variables';
 
 .app-body{
- -ms-overflow-style: -ms-autohiding-scrollbar; 
+ -ms-overflow-style: -ms-autohiding-scrollbar;
 }
 
 .button {
@@ -165,6 +165,14 @@
   }
 }
 
+.avatar {
+  border-radius: 4px;
+  background: transparent no-repeat;
+  background-position: 50%;
+  background-clip: padding-box;
+  position: relative;
+}
+
 .lightbox .icon-button {
   color: $color1;
 }