Add support for email alerts on new conversations only (not all new messages)

[reichert621]

Problem
Our email notifications right now can either do all new messages, or none at all.

Solution
Some users may want to only be notified when a new conversation is started.

• Add new field to user_settings: email_alert_on_new_conversation (default false)
• Add new checkbox to user profile UI
• Update the ChatApi.Emails.send_new_message_alerts method to take this new field into account

Testing
Please include automated tests in your PR.

Questions, or need help getting started?
Feel free to ask below, or ping us on Slack :)

(You can also check out our CONTRIBUTING.md)