Rate-limit emails sent by the same user in a period of time

[mtomilov]

Fixes #9411

Here we rate-limit emails sent by a single user to 100 within the last 24h to prevent abuse.

Testing

• Set DAILY_SENDER_MENTION_LIMIT to a lower value like 5
• Log in as devdata_admin
• Create 6 annotations mentioning devdata_user
• Observe that only 5 emails are present in the mail folder
• Observe the limit reached line in celery logs

[acelaya]

Looks good!

I tested it by setting DAILY_SENDER_MENTION_LIMIT = 3. After the third mention, I was not able to send more, and the warning message was logged