Add letter validation preview functionality

- add get/post view
- create a pdf upload form
- add a template where user can upload the file
- check boundaries of the letter by calling template-preview
- display banner messages with boundaries validation result
- display pages of the document, with visible boundaries overlay
if the document did not pass validation, and without overlay
if they do pass validation

app/main/forms.py

@@ -766,6 +766,13 @@ class ServiceUpdateEmailBranding(StripWhitespaceForm):
     )
 
 
+class PDFUploadForm(StripWhitespaceForm):
+    file = FileField_wtf(
+        'Upload a letter in PDF format to check if it fits in the printable area',
+        validators=[FileAllowed(['pdf'], 'PDF documents only!')]
+    )
+
+
 class CreateOrUpdateOrganisation(StripWhitespaceForm):
 
     name = StringField('Name', validators=[DataRequired()])