Sprinkle letter throughout the app

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.

app/assets/stylesheets/main.scss

@@ -58,6 +58,7 @@ $path: '/static/images/';
 @import 'components/research-mode';
 @import 'components/tick-cross';
 @import 'components/list-entry';
+@import 'components/letter';
 
 @import 'views/job';
 @import 'views/edit-template';