Add proper email notifications

templates/emails/verifying.body.txt

@@ -0,0 +1,18 @@
+Hi,
+
+Good news -- your hamprint submission "{{ submission.slug }}" passed the
+automated checks (size, format, host allow-list) and is now waiting for an
+operator to take a manual look.
+
+    Codename : {{ submission.slug }}
+    Status   : Verifying
+
+If the operator is happy with the print job, it moves into the print queue
+shortly after and we'll email you again. You don't need to do anything
+right now.
+
+You can also check on it here:
+    {{ detail_url }}
+
+— hamprint
+{{ site_url }}