Add simple_field() def for base falafel template

hopefully this is a useful abstraction which will allow for smoother transition
to Buefy-style fields, when the time comes?

tailbone/templates/themes/falafel/base.mako

@@ -363,3 +363,16 @@
     </div>
   </div>
 </%def>
+
+<%def name="simple_field(label, value)">
+  ## TODO: keep this? only used by personal profile view currently
+  ## (although could be useful for any readonly scenario)
+  <div class="field-wrapper">
+    <div class="field-row">
+      <label>${label}</label>
+      <div class="field">
+        ${'' if value is None else value}
+      </div>
+    </div>
+  </div>
+</%def>