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/structures/HomePage.js

@@ -108,7 +108,7 @@ class HomePage extends React.Component {
         if (this.context.matrixClient.isGuest()) {
             guestWarning = (
                 <div className="mx_HomePage_guest_warning">
-                    <img src="img/warning.svg" width="24" height="23" />
+                    <img src={require("../../../res/img/warning.svg")} width="24" height="23" />
                     <div>
                         <div>
                             { _t("You are currently using Riot anonymously as a guest.") }