[feature] Email notifications for new / closed moderation reports

[tsmethurst]

Description

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This pull request implements email notifications for:

• all instance admins+moderators when a new report is created
• creator of the report (local account only) when their report is closed

I used the gts account to ask folks whether they would want emails to multiple admins to be Bcc'd or To'd, and the results were quite mixed. Some people prefer anonymous notifications, and some people suggested it would be nice to be able to use reply-all to discuss reports with other admins. To serve both those needs, there's now a config flag smtp-disclose-recipients which, when enabled, puts all admins + moderators in To instead of Bcc.

Examples:

direct email To:

Email to Undisclosed Recipients:

Email to a user when their report has been closed by a moderator:

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.