Warn users a template change will break things

When a user adds or removes placeholders in their template we should consider
this a ‘breaking change’ and warn them accordingly.

Implementing this mostly relies on using
https://github.com/alphagov/notifications-utils/pull/37

Temporarily storing the new template until the user confirms that they want to
make the changes in done using hidden fields. This is a bit hacky, but the
complexity of making sessions interact with WTForms was just too much to handle.

This commit also changes the example spreadsheet that we show on this page to
look more like a spreadsheet.

app/assets/stylesheets/app.scss

@@ -153,3 +153,24 @@ details summary {
   text-decoration: underline;
   margin-bottom: $gutter-half;
 }
+
+.spreadsheet {
+
+  th,
+  .table-field-index {
+    background: $grey-4;
+    font-weight: bold;
+    text-align: center;
+  }
+
+  th, td {
+    padding-left: 10px;
+    padding-right: 10px;
+    border: 1px solid $border-colour;
+  }
+
+  td {
+    border-top: 0;
+  }
+
+}