Improve automatic emails with temporary urls

This MR targets the improvement for automatic emails concerning validation of new users by supervisors/re-validation of users.

Fixes #542 (closed)

Fixes #550 (closed)

beat/web/accounts/models.py

@@ -181,10 +181,12 @@ class TemporaryUrl(models.Model):
     # Self yearly revalidation from supervisee
     VALIDATION = "V"
     YEARREVALIDATION = "Y"
+    REJECTION = "R"
 
     URL_STATUS = (
         (VALIDATION, "Supervisor validates supervisee"),
         (YEARREVALIDATION, "Self yearly revalidation from supervisee"),
+        (REJECTION, "Supervisor rejects supervisee revalidation"),
     )
 
     # _____ Fields __________