* remove email verification related code
* remove email verification related code
* remove sendgrid callback
* remove sendgrid related code
* remove sendgrid related code
* rename sendgrid app to email
* remove email from built-in channels
* remove email from built-in channels
* remove email from built-in channels
* add email backend: https://github.com/grafana/oncall/pull/50
* add email templater
* add email templater
* convert md to html
* add email settings to live settings
* use task to send email, handle some exceptions to create logs
* remove ERROR_NOTIFICATION_MAIL_DELIVERY_FAILED usage
* add email limit logic
* fix tests
* add docs
* remove old email templates
* remove old email templates
* add template_fields to messaging backend
* add messaging backends templates to public api
* add comment for deprecated fields
* fix test
* fix tests
* disable email by default
* don't retry on SMTPException and TimeoutError
* add tests
* bring email back to public api docs
* return ERROR_NOTIFICATION_MAIL_LIMIT_EXCEEDED
* make template_fields tuple
* build_subject_and_title -> build_subject_and_message
* add one more comment about template deprecation
* use 8 as backend id
* add comment about gaierror and BadHeaderError
* add comment on importing in notify_user_async
* edit oss docs
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