Add letter upload form which redirects to blank preview page

Added a form to upload a single letter. Currently this only uses the
form to validate that a file is submitted and that the file is a PDF. If
either of these validations fail, the form will display an error.
Otherwise, we redirect to a new preview page which just has the filename
as the heading for now.

app/main/forms.py

@@ -1109,9 +1109,9 @@ class ServiceLetterBrandingDetails(StripWhitespaceForm):
 
 class PDFUploadForm(StripWhitespaceForm):
     file = FileField_wtf(
-        'Upload a letter in PDF format to check if it fits in the printable area',
+        'Upload a letter in PDF format',
         validators=[
-            FileAllowed(['pdf'], 'PDF documents only!'),
+            FileAllowed(['pdf'], 'Letters must be saved as a PDF'),
             DataRequired(message="You need to upload a file to submit")
         ]
     )