Wrap unsupported form field notes in proper divs (thoughtbot#515)

* Wrap unsupported form field notes in proper divs

Problem:

Forms for `has_one` and `polymorphic` relationships
aren't currently supported,
and they show up on form pages as a note to the user.

This note is misaligned with the other form fields,
because it is not wrapped in the same div
that applies to the other fields.

Solution:

Add the apropriate `field-unit__field` div
to the unsupported field types.

* Fix syntax error in ERB

app/views/fields/has_one/_form.html.erb

@@ -21,4 +21,6 @@ so this partial renders a message to that effect.
   <%= f.label field.attribute %>
 </div>
 
-<%= t("administrate.fields.has_one.not_supported") %>
+<div class="field-unit__field">
+  <%= t("administrate.fields.has_one.not_supported") %>
+</div>

app/views/fields/polymorphic/_form.html.erb

@@ -22,4 +22,6 @@ so this partial renders a message to that effect.
   <%= f.label field.name %>
 </div>
 
-<%= t("administrate.fields.polymorphic.not_supported") %>
+<div class="field-unit__field">
+  <%= t("administrate.fields.polymorphic.not_supported") %>
+</div>