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/elements/SettingsButton.js

@@ -24,7 +24,7 @@ const SettingsButton = function(props) {
     return (
         <ActionButton action="view_user_settings"
             label={_t("Settings")}
-            iconPath="img/icons-settings.svg"
+            iconPath={require("../../../../res/img/icons-settings.svg")}
             size={props.size}
             tooltip={props.tooltip}
         />