embed mode: force embed-mode class via JS since Jinja2 context not flowing

The embed_mode variable set in board.py context wasn't reaching
base.html through Jinja2 extends. Added document.body.classList.add
in page_editor_embed.html content block to force the class, which
triggers the CSS rules hiding sidebar, topbar, and header.

app/templates/page_editor_embed.html

@@ -4,6 +4,7 @@
 {% block topbar %}{% endblock %}
 {% block content %}
 {% include '_page_editor_content.html' %}
+<script>document.body.classList.add('embed-mode');</script>
 {% endblock %}
 {% block scripts %}
 <script type="application/json" id="page-data">