Fix missing CSRF token on send SMS page

As part of https://github.com/alphagov/notifications-admin/pull/187 the file
upload pattern was changed to auto-submit once a file had been picked. The
form that was submitted was, however, missing a CSRF token, as well as a submit
button for non-JS users.

This commit makes the file upload pattern self-contained, so that it will always
include a form with a CSRF token in a hidden input and a submit button, which is
then hidden when Javascript loads.

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

@@ -3,9 +3,15 @@
   .file-upload {
 
     &-label {
+
       @include bold-19;
       display: block;
       margin: 0 0 10px 0;
+
+      .error-message {
+        padding: 0;
+      }
+
     }
 
     &-field {
@@ -33,6 +39,10 @@
       padding-left: $gutter-half;
     }
 
+    &-submit {
+      display: none;
+    }
+
   }
 
 }