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/groups/GroupMemberInfo.js

@@ -188,7 +188,7 @@ module.exports = React.createClass({
             <div className="mx_MemberInfo">
                 <GeminiScrollbarWrapper autoshow={true}>
                     <AccessibleButton className="mx_MemberInfo_cancel" onClick={this._onCancel}>
-                        <img src="img/cancel.svg" width="18" height="18" className="mx_filterFlipColor" />
+                        <img src={require("../../../../res/img/cancel.svg")} width="18" height="18" className="mx_filterFlipColor" />
                     </AccessibleButton>
                     <div className="mx_MemberInfo_avatar">
                         { avatar }