Move verse selector to htmx

assets/templates/verses.njk

@@ -0,0 +1,7 @@
+{# The verse select's options. Rendered into the picker by reference.njk, and
+   returned on their own by api/reference after a chapter or verse change.
+   Both paths must produce the same options for the swap to be seamless. #}
+{% for verse in reference.verses -%}
+    <option value="{{ verse.id }}"
+            {%- if verse.id == reference.verse %} selected{% endif %}>{{ verse.label }}</option>
+{% endfor -%}