Add a hint about how to use placeholders

Since placeholders (almost) work now, it’s worth telling people what the syntax
is.

This commit also removes the ‘template type’ picker, since you can only create
SMS templates at the moment. This will be revisited when we start looking at how
you add an email template.

app/templates/views/edit-template.html

@@ -11,17 +11,22 @@
     <h1 class="heading-large">{{ h1 }}</h1>
 
     <form method="post">
-      {{ textbox(form.name) }}
-      <fieldset class="form-group">
-        <legend class="form-label">
-          Template type
-        </legend>
-        <label class="block-label" for="template_type">
-          <input type="radio" name="template_type" id="template_type" checked="checked" value="sms" />
-          Text message
-        </label>
-      </fieldset>
-      {{ textbox(form.template_content, highlight_tags=True) }}
+      <div class="grid-row">
+        <div class="column-two-thirds">
+          {{ textbox(form.name, width='1-1') }}
+        </div>
+      </div>
+      <div class="grid-row">
+        <div class="column-two-thirds">
+          {{ textbox(form.template_content, highlight_tags=True, width='1-1') }}
+        </div>
+        <div class="column-one-third">
+          <label for='template_content' class='edit-template-placeholder-hint'>
+            Add placeholders using double brackets, eg Your thing
+            is due on ((date))
+          </label>
+        </div>
+      </div>
       {{ page_footer(
         'Save',
         delete_link=url_for('.delete_service_template', service_id=service_id, template_id=template_id) if template_id or None,