XWIKI-19294: Fix bad escaping

(cherry picked from commit bd935320)

xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-theme/xwiki-platform-flamingo-theme-ui/src/main/resources/FlamingoThemesCode/WebHomeSheet.xml

@@ -287,7 +287,7 @@
       <form action="$doc.getURL()" method="post">
         <input type="hidden" name="form_token" value="$services.csrf.token" />
         <input type="hidden" name="action" value="create"/>
-        <input type="hidden" name="newThemeName" id="newThemeName" value="$request.newThemeName" />
+        <input type="hidden" name="newThemeName" id="newThemeName" value="$escapetool.xml($request.newThemeName)" />
         <input type="submit" value="$services.localization.render('platform.flamingo.themes.home.create.confirm')" class="button"/>
       </form>
     {{/html}}