Convert image URLs in React to require calls

This allows Webpack to insert the proper image URL after builds steps like
adding a hash and so on. The path you supply to `require` is relative to the JS
source file, just like any other would be.

src/components/views/rooms/MemberList.js

@@ -253,7 +253,7 @@ module.exports = React.createClass({
         const text = _t("and %(count)s others...", { count: overflowCount });
         return (
             <EntityTile className="mx_EntityTile_ellipsis" avatarJsx={
-                <BaseAvatar url="img/ellipsis.svg" name="..." width={36} height={36} />
+                <BaseAvatar url={require("../../../../res/img/ellipsis.svg")} name="..." width={36} height={36} />
             } name={text} presenceState="online" suppressOnHover={true}
             onClick={onClick} />
         );