feat: source_format selector on new-thread create form

The preview fix (64a15b6) honored source_format only when the textarea
already declared one — which works for editing an existing node but
not for creating a new one. Authors writing RST in the create form
saw their preview rendered as markdown, then saved into a node whose
source_format defaulted to markdown — so even the final saved page
came back wrong.

Three pieces:

- create.j2: <select> for source_format alongside the textarea.
  Default markdown; rst, html, mediawiki, latex, org, asciidoc,
  textile available. onchange syncs the chosen value into
  textarea.dataset.sourceFormat and re-runs preview, so the
  existing custom.js sendPreview picks it up unchanged.

- new_thread.py: read source_format from request.params, pass to
  set_data() so the created node's persistent source_format matches
  what the author saw in preview.

- No JS change needed — custom.js already reads
  textarea.dataset.sourceFormat.

remarkbox/templates/snippets/create.j2

@@ -11,10 +11,24 @@
         name        = "thread_data"
         id          = "thread_data_textarea"
         class       = "common-textarea"
+        data-source-format = "markdown"
         onkeyup     = "previewAjax( 'thread_data_textarea', 'preview', show_raw=true, mathjax={{ request.mathjax }} )"
         placeholder = "What do you want to say? markdown"
         required></textarea>
 
+  <label for="source_format_select" style="display:inline-block;margin-right:0.5em">format:</label>
+  <select id="source_format_select" name="source_format"
+          onchange="document.getElementById('thread_data_textarea').dataset.sourceFormat = this.value; previewAjax('thread_data_textarea', 'preview', show_raw=true, mathjax={{ request.mathjax }})">
+    <option value="markdown" selected>markdown</option>
+    <option value="rst">reStructuredText</option>
+    <option value="html">html</option>
+    <option value="mediawiki">mediawiki</option>
+    <option value="latex">latex</option>
+    <option value="org">org</option>
+    <option value="asciidoc">asciidoc</option>
+    <option value="textile">textile</option>
+  </select>
+
   <div id='preview' name='preview' class="preview"></div>
 
 {% include 'csrf.j2' %}

remarkbox/views/new_thread.py

@@ -22,6 +22,7 @@ def new_thread(request):
     thread_title = request.params.get("thread_title", "")
     thread_data = request.params.get("thread_data", "")
     anonymous_name = request.params.get("anonymous_name", "").strip()
+    source_format = request.params.get("source_format", "").strip() or None
 
     if request.spam:
         return request.spam
@@ -61,7 +62,7 @@ def new_thread(request):
         node.namespace = request.namespace
         node.ip_address = unicode(request.client_addr)
         node.title = thread_title
-        node.set_data(thread_data, dbsession=request.dbsession)
+        node.set_data(thread_data, dbsession=request.dbsession, source_format=source_format)
 
         # Handle anonymous vs authenticated user
         if user_surrogate: