feat: add button to export the rst document as PDF

Add button to export the rst document as PDF.
As requested here, anru#28 .
Most of the code was already there, I refactored and did small changes.
Give the `textarea` the `form` attibute to attach it to the form id
`save_as_pdf`.

templates/index.html

@@ -36,24 +36,25 @@
 {% block content %}
 <div class="b_page_cont">
     <div class="b_page_left">
-        <textarea id="editor" rows="15">{{ rst }}</textarea>
+        <textarea id="editor" rows="15" name="text" form="save_as_pdf">{{ rst }}</textarea>
     </div>
     <div class="b_page_right">
         <div class="right_nav">
             <div class="themes left" id="themes">Theme:
                 <input id="t_basic" type="radio" value="basic" checked="checked" name="style"/><label for="t_basic">Basic</label>
                 <input id="t_nature" type="radio" value="nature" name="style"/><label for="t_nature">Nature</label>
             </div>
+        </div>
+        <div class="right">
+            <form id="save_as_pdf" method="POST" action="{{ request.script_root }}/srv/rst2pdf/">
+                <input type="submit" id="as_pdf_rst" name="rst" value="Export to PDF"/>
+                <input type="hidden" id="as_pdf_theme" name="theme"/>
+            </form>
         </div>
 	    <iframe src="{{ request.script_root }}/srv/rst2html/" id="browse"></iframe>
     </div>
 </div>
 
 </div>
 
-<form id="save_as_pdf" method="POST" action="{{ request.script_root }}/srv/rst2pdf/">
-    <input type="hidden" id="as_pdf_rst" name="rst" />
-    <input type="hidden" id="as_pdf_theme" name="theme"/>
-</form>
-
 {% endblock %}