feat: identity linked/unlinked notifications

[issuedat]

This PR adds support for sending email notifications to a user when an identity has been linked/unlinked for their account.

6 new environment variables are introduced:

• GOTRUE_MAILER_SUBJECTS_IDENTITY_{LINKED|UNLINKED}_NOTIFICATION: Email subject to use for identity linked/unlinked notification.
• GOTRUE_MAILER_TEMPLATES_IDENTITY_{LINKED|UNLINKED}_NOTIFICATION: The URL to specify a custom template.
• GOTRUE_MAILER_NOTIFICATIONS_IDENTITY_{LINKED|UNLINKED}_ENABLED: whether the notification is enabled or not.

The feature is disabled by default. To enable it, the GOTRUE_MAILER_NOTIFICATIONS_IDENTITY_{LINKED|UNLINKED}_ENABLED environment variable must be set to true.

[coveralls]

Pull Request Test Coverage Report for Build 18225834863

Details

• 46 of 84 (54.76%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 67.554%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/verify.go	3	7	42.86%
internal/api/identity.go	5	11	45.45%
internal/api/mail.go	18	30	60.0%
internal/mailer/templatemailer/templatemailer.go	0	16	0.0%

Totals
Change from base Build 18170389302:	-0.04%
Covered Lines:	13429
Relevant Lines:	19879

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💛 - Coveralls