Add an ‘add logo’ button

Because some people don’t know they can put their own logo on letters:

> The HM Government Logo is at the top of the letter and we can't see
> a way of putting the [organisation] logo on

> We are intending to use the letter template feature for the first time
> and wondered whether the branding is configurable or whether the HM
> Government header is the standard default.

> Can we replace HM Government logo with our own in the letter? IF yes,
> then how?

> I don't seem to be able to set the branding on the letters to be
> [organisation]. it's always HM government. Is there something that
> needs enabling for this account?

No-one actually wants the HM Government logo (no-one is sending real
letters using it). So we should leave the space blank and put a button
there prompting people to add their own logo.

app/main/views/service_settings.py

@@ -829,6 +829,7 @@ def service_set_letter_branding(service_id):
 def request_letter_branding(service_id):
     return render_template(
         'views/service-settings/request-letter-branding.html',
+        from_template=request.args.get('from_template'),
     )