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")
         ]
     )

app/main/views/uploads.py

@@ -1,6 +1,10 @@
-from flask import render_template
+import uuid
 
+from flask import redirect, render_template, request, url_for
+
+from app import current_service
 from app.main import main
+from app.main.forms import PDFUploadForm
 from app.utils import user_has_permissions
 
 
@@ -8,3 +12,31 @@ from app.utils import user_has_permissions
 @user_has_permissions('send_messages')
 def uploads(service_id):
     return render_template('views/uploads/index.html')
+
+
+@main.route("/services/<service_id>/upload-letter", methods=['GET', 'POST'])
+@user_has_permissions('send_messages')
+def upload_letter(service_id):
+    form = PDFUploadForm()
+
+    if form.validate_on_submit():
+        upload_id = uuid.uuid4()
+
+        return redirect(
+            url_for(
+                'main.uploaded_letter_preview',
+                service_id=current_service.id,
+                file_id=upload_id,
+                original_filename=form.file.data.filename,
+            )
+        )
+
+    return render_template('views/uploads/choose-file.html', form=form)
+
+
+@main.route("/services/<service_id>/preview-letter/<file_id>")
+@user_has_permissions('send_messages')
+def uploaded_letter_preview(service_id, file_id):
+    original_filename = request.args.get('original_filename')
+
+    return render_template('views/uploads/preview.html', original_filename=original_filename)