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/AddressSelector.js

@@ -150,7 +150,7 @@ export default React.createClass({
                             showAddress={this.props.showAddress}
                             justified={true}
                             networkName="vector"
-                            networkUrl="img/search-icon-vector.svg"
+                            networkUrl={require("../../../../res/img/search-icon-vector.svg")}
                         />
                     </div>,
                 );