Show one field at a time on send yourself a test

The send yourself a test feature is useful for two things:
- constructing an email/text message/letter without uploading a CSV file
- seeing what the thing your going to send will look like (either by
  getting it in your inbox or downloading the PDF)
- learning the concept of placeholders, ie understanding they’re thing
  that gets populated with _stuff_

The problem we’re seeing is that the current UI breaks when a template
has a lot of placeholders. This is especially apparent with letter
templates, which have a minimum of 7 placeholders by virtue of the
address.

The idea behind having the form fields side-by-side was to help people
understand the relationship between their spreadsheet columns and the
placeholders. But this means that the page was doing a lot of work,
trying to teach:
- replacement of placeholders
- link between placeholders and spreadsheet columns

The latter is better explained by the example spreadsheet shown on the
upload page. So it can safely be removed from the send yourself a test
page – in other words the fields don’t need to be shown side by side.

Showing them one-at-a-time works well because:
- it’s really obvious, even on first use, what the page is asking you to
  do
- as your step through each placeholder, you see the message build up
  with the data you’ve entered – you’re learning how replacement of
  placeholders works by repetition

This also means adding a matching endpoint for viewing each step of
making the test letter as a PDF/PNG because we can’t reuse the view of
the template without any placeholders filled any more.

app/utils.py

@@ -223,6 +223,15 @@ class Spreadsheet():
                 output.writerow(row)
             return cls(converted.getvalue(), filename)
 
+    @classmethod
+    def from_dict(cls, dictionary, filename=''):
+        return cls.from_rows(
+            zip(
+                *sorted(dictionary.items(), key=lambda pair: pair[0])
+            ),
+            filename
+        )
+
     @classmethod
     def from_file(cls, file_content, filename=''):
         extension = cls.get_extension(filename)