use newer email API and send with UTF-8 support

compendium_v2/email/__init__.py

 import smtplib
 import threading
 import logging
+from email.mime.multipart import MIMEMultipart
+from email.mime.text import MIMEText
 from typing import Sequence, Union
 from sqlalchemy import select
 from flask import current_app
@@ -22,11 +24,11 @@ Daniel and Jennifer (the Compendium admins)
 """  # noqa: E501
 
 
-def _send_mail(smtp_server, port, sender_email, recipients, message):
+def _send_mail(smtp_server: str, port: int, recipients: Union[str, Sequence[str]], message: MIMEMultipart):
 
     try:
         with smtplib.SMTP(smtp_server, port) as server:
-            server.sendmail(from_addr=sender_email, to_addrs=recipients, msg=message)
+            server.sendmail(from_addr=message['From'], to_addrs=recipients, msg=message.as_string())
         logger.debug('Successfully sent email')
     except Exception:
         logger.exception('Unable to send email:')
@@ -53,16 +55,18 @@ def send_mail(
     if not recipients:
         recipients = admin_emails
 
-    subject = subject.replace('\n', ' ')
-    message = f"""Subject: {subject}\n\n{contents}"""
+    message = MIMEMultipart('alternative')
+    message['Subject'] = subject
+    message['From'] = current_app.config['MAIL_SENDER_EMAIL']
+    message['To'] = ', '.join(recipients)
+    message.attach(MIMEText(contents, 'plain', 'utf-8'))
 
     smtp_server = current_app.config['MAIL_SERVER']
     port = current_app.config['MAIL_PORT']
-    sender_email = current_app.config['MAIL_SENDER_EMAIL']
 
     # spin off a thread since this can take some time..
     logger.debug('Sending email')
-    thread = threading.Thread(target=_send_mail, args=(smtp_server, port, sender_email, recipients, message))
+    thread = threading.Thread(target=_send_mail, args=(smtp_server, port, recipients, message))
     thread.start()