* 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
