preserve previous state on form error

the html now contains a `data-prev-state` attribute which contains the
previous state, taken from the `operation` value in the form data (from
the submit button). This is used to seed the `currentState` of the
templateFolderForm. If not specified (or 'unknown', because the user
hit enter last time round), then set it to nothingSelectedButtons.

app/templates/components/form.html

@@ -4,7 +4,8 @@
   autocomplete=False,
   class=None,
   id=None,
-  module=None
+  module=None,
+  data_kwargs={}
 ) %}
   <form
     method="{{ method }}"
@@ -13,6 +14,11 @@
     {% if class %}class="{{ class }}"{% endif %}
     {% if id %}id="{{ id }}"{% endif %}
     {% if module %}data-module="{{ module }}"{% endif %}
+    {% for key, val in data_kwargs.items() %}
+      {% if val %}
+        data-{{ key }}="{{ val }}"
+      {% endif %}
+    {% endfor %}
     novalidate
   >
     {{ caller() }}