Make send a test teach you how placeholders work

This commit does two main things:

- adds textboxes to the send yourself a test page, so you can replace
  ((name)) with ‘Chris’, or whatever your name is
- rejigs the send page a bit to make it clearer what the CSV upload is
  for and how to use it

The idea being that, since most users go into ‘send yourself a test’
first, it teaches them about how placeholders work by making them do the
replacing themselves.

app/assets/stylesheets/components/file-upload.scss

@@ -50,6 +50,11 @@
       display: none;
     }
 
+    &-alternate-link {
+      display: inline-block;
+      line-height: 35px;
+    }
+
   }
 
 }

app/assets/stylesheets/components/page-footer.scss

@@ -5,7 +5,6 @@
   &-back-link {
     @include button($grey-1);
     display: inline-block;
-    margin-left: 10px;
   }
 
   &-delete-link {
@@ -36,6 +35,11 @@
     margin-top: $gutter;
   }
 
+  .button,
+  .button-destructive {
+    margin-right: 10px;
+  }
+
   .button-destructive {
     @include button($error-colour);
     padding: 0.52632em 0.78947em 0.26316em 0.78947em;