Fix spacing of first page headers

This commit customises the margins of the first header on each page so that it
lines up with the navigation.

app/assets/stylesheets/app.scss

@@ -38,3 +38,11 @@ a:visited {
     width: 5em;
   }
 }
+
+.column-main {
+
+  > .heading-large {
+    margin-top: $gutter;
+  }
+
+}

app/templates/views/choose-sms-template.html

@@ -8,9 +8,10 @@
 {% endblock %}
 
 {% block maincolumn_content %}
-  <form method="POST" enctype="multipart/form-data">
 
-    <h1 class="heading-large">Send text messages</h1>
+  <h1 class="heading-large">Send text messages</h1>
+
+  <form method="POST" enctype="multipart/form-data">
 
     {% if templates %}
       <fieldset class='form-group'>

app/templates/views/documentation.html

@@ -8,13 +8,13 @@ GOV.UK Notify | API keys and documentation
 
 {% block maincolumn_content %}
 
+<h1 class="heading-large">
+    Developer documentation
+</h1>
+
 <div class="grid-row">
     <div class="column-two-thirds">
 
-        <h1 class="heading-large">
-            Developer documentation
-        </h1>
-
         <h2 class="heading-medium">
             How to integrate GOV.UK Notify into your service
         </h2>

app/templates/withnav_template.html

@@ -5,7 +5,7 @@
     <div class="column-one-third">
       {% include "main_nav.html" %}
     </div>
-    <div class="column-two-thirds">
+    <div class="column-two-thirds column-main">
       {% include 'flash_messages.html' %}
       {% block maincolumn_content %}{% endblock %}
     </div>