Feature/comp 257 send email on signup

compendium_v2/email/__init__.py

@@ -29,14 +29,8 @@ def send_mail(
         logger.warning('No mail configuration, cannot send email.')
         return
 
-    if not isinstance(contents, str):
-        raise ValueError('Contents must be a string.')
-
-    if not isinstance(subject, str):
-        raise ValueError('Subject must be a string.')
-
-    if not isinstance(recipients, (str, Sequence)):
-        raise ValueError('Recipients must be a string or a sequence of strings.')
+    if not contents or not isinstance(contents, str):
+        raise ValueError('Contents must be a non-empty string.')
 
     excluded_admins = set(email.lower() for email in current_app.config['MAIL_EXCLUDED_ADMINS'])