Update MarkdownSectionController with support for editing

This allows the MarkdownSectionController and ItemPageController to
work together to begin an editing session for an item description.

src/jade/templates/markdown_section.jade

@@ -8,11 +8,11 @@
 .view__markdown_section.section
     h2
     .panel
-        .markdown.hideable
+        .waiting.hideable: img(src='/images/wait.gif')
+        .markdown.hideable.hidden
         .editor.hideable.hidden
             textarea
             .sizer
-        .images.hideable.hidden
         .buttons.hideable.hidden
             .error.hideable.hidden: p
             button.return.hideable.hidden return
@@ -20,3 +20,9 @@
             button.preview.hideable.hidden preview
             button.save.hideable.hidden save
             button.edit.hideable edit
+        .confirming.hideable.hidden
+            p Thanks for your help!
+            p Your changes will appear on the site in a few minutes.
+        .appologizing.hideable.hidden
+            p Sorry, but something went wrong.
+            p Please try again after a few moments.