Fix mail thread grouping for separate check runs

Don't keep In-Reply-To between success/failure cycles not to have all
reports from a particular check in the same thread.

Fixes a bug introduced in the previous commit,
3c3ca1cf

picomon/mails.py

@@ -135,7 +135,7 @@ def send_email_for_check(check, removed=False):
     extra_headers = {}
     extra_headers['Message-ID'] = make_msgid(type(check).__name__)
     # if it's a follow up, set In-Reply-To
-    if hasattr(check, 'mails_msgid'):
+    if (check.ok or removed) and hasattr(check, 'mails_msgid'):
         extra_headers['In-Reply-To'] = check.mails_msgid
         extra_headers['References'] = check.mails_msgid
     check.mails_msgid = extra_headers['Message-ID']