Hide the instructions for formatting a CSV

> On the send pages, we have a call to action in the middle of the
> page with lots of words and placeholder buttony looking things
> above and below it....
>
> You'll also see this every time you use this page, even though you
> probably get it after a single use.
>
> So let's wrap it up under a usefully titled (?) help link that
> expands to reveal all the things.

This commit implements the above.

It also rewords the messaging to talk about various spreadsheet formats,
not just CSV.

app/assets/stylesheets/app.scss

@@ -61,11 +61,11 @@ a {
   }
 
   > .grid-row  {
-    
+
     .heading-large {
       margin-top: $gutter;
     }
-    
+
   }
 
 }
@@ -104,7 +104,7 @@ td {
 #footer {
 
   .footer-categories {
-    
+
     @extend %site-width-container;
 
     &-wrapper {
@@ -148,3 +148,8 @@ a[rel='external'] {
   }
 
 }
+
+details summary {
+  text-decoration: underline;
+  margin-bottom: $gutter-half;
+}