✨(backend) email invitation to new users

When generating an Invitation object within the database, our intention
is to promptly notify the user via email. We send them an invitation
to join Desk.

This code is inspired by Joanie successful order flow.

Johann's design was missing a link to Desk, I simply added a button which
redirect to the staging url. This url is hardcoded, we should refactor it
when we will deploy Desk in pre-prod or prod environments.

Johann's design relied on Marianne font. I implemented a simpler version,
which uses a google font. That's not important for MVP.

Look and feel of this first invitation template is enough to make our PoC
functionnal, which is the more important.

src/backend/debug/urls.py

@@ -9,13 +9,13 @@ from .views import (
 
 urlpatterns = [
     path(
-        "__debug__/mail/hello_html",
+        "__debug__/mail/invitation_html",
         DebugViewHtml.as_view(),
-        name="debug.mail.hello_html",
+        name="debug.mail.invitation_html",
     ),
     path(
-        "__debug__/mail/hello_txt",
+        "__debug__/mail/invitation_txt",
         DebugViewTxt.as_view(),
-        name="debug.mail.hello_txt",
+        name="debug.mail.invitation_txt",
     ),
 ]

src/backend/debug/views.py

@@ -11,8 +11,6 @@ class DebugBaseView(TemplateView):
 
         context = super().get_context_data(**kwargs)
         context["title"] = "Development email preview"
-        context["email"] = "random@gmail.com"
-        context["fullname"] = "robert"
 
         return context
 
@@ -20,10 +18,10 @@ class DebugBaseView(TemplateView):
 class DebugViewHtml(DebugBaseView):
     """Debug View for HTML Email Layout"""
 
-    template_name = "mail/html/hello.html"
+    template_name = "mail/html/invitation.html"
 
 
 class DebugViewTxt(DebugBaseView):
     """Debug View for Text Email Layout"""
 
-    template_name = "mail/text/hello.txt"
+    template_name = "mail/text/invitation.txt"