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/dialogs/BaseDialog.js

@@ -111,7 +111,7 @@ export default React.createClass({
         let cancelButton;
         if (this.props.hasCancel) {
             cancelButton = <AccessibleButton onClick={this._onCancelClick} className="mx_Dialog_cancelButton">
-                <TintableSvg src="img/icons-close-button.svg" width="35" height="35" />
+                <TintableSvg src={require("../../../../res/img/icons-close-button.svg")} width="35" height="35" />
             </AccessibleButton>;
         }