Let users create/edit/delete letter templates.
Let them upload a CSV file or send a test against a letter template.
Big assumption at the moment is that addresses only have one line, and
therefore one column in the CSV file.
For the button on the check page, we need to be able to say ‘1 text message’ or
‘55 emails’. We already have the logic to do this on the dashboard (101 text
messages sent), and it’s already in a component. So this commit makes the check
page use the same component.