Add an endpoint for generating a PDF of a letter

Previewing a letter is different to previewing an email or text message
because:

- a letter has a layout
- the layout is fixed, ie it doesn’t depend on the user’s device
- the ‘send yourself a test’ feature won’t be as useful because it has
  a lead time, so the feedback loop will be much longer

For these reasons a HTML-only preview of the letter won’t be enough (we
don’t think). A PDF is more appropriate because:

- it can replicate the layout of the letter exactly
- it is a print format, so the user could even print themselves a copy
  locally to get a feel for how it will look

This commit makes use of Flask WeasyPrint [1] to take a HTML
representation of the letter, convert it to a PDF and serve it back to
the user.

The actual work to generate the HTML and specify the layout is done in
utils, same as we do for rendering other messages.

1. https://pythonhosted.org/Flask-WeasyPrint/

app/main/views/templates.py

@@ -3,10 +3,12 @@ from string import ascii_uppercase
 
 from flask import request, render_template, redirect, url_for, flash, abort
 from flask_login import login_required
+from flask_weasyprint import HTML, render_pdf
 from dateutil.parser import parse
 
 from notifications_utils.template import Template
 from notifications_utils.recipients import first_column_headings
+from notifications_utils.renderers import LetterPreview
 from notifications_python_client.errors import HTTPError
 
 from app.main import main
@@ -51,6 +53,17 @@ def view_template(service_id, template_id):
     )
 
 
+@main.route("/services/<service_id>/templates/<template_id>.pdf")
+@login_required
+@user_has_permissions('view_activity', admin_override=True)
+def view_letter_template_as_pdf(service_id, template_id):
+    template = Template(
+        service_api_client.get_service_template(service_id, template_id)['data'],
+        renderer=LetterPreview()
+    )
+    return render_pdf(HTML(string=template.rendered))
+
+
 @main.route("/services/<service_id>/templates/<template_id>/version/<int:version>")
 @login_required
 @user_has_permissions(