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/AppWarning.js

@@ -5,7 +5,7 @@ const AppWarning = (props) => {
     return (
         <div className='mx_AppPermissionWarning'>
             <div className='mx_AppPermissionWarningImage'>
-                <img src='img/warning.svg' alt='' />
+                <img src={require("../../../../res/img/warning.svg")} alt='' />
             </div>
             <div className='mx_AppPermissionWarningText'>
                 <span className='mx_AppPermissionWarningTextLabel'>{ props.errorMsg }</span>