fix: pass ServerName in tls config to send mails in secure mode

[anujk14]

Bug description

If app.mailer.smtp_insecure is set to false in the config, Frontier is unable to send emails, and we get the following error: tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config.

When app.mailer.smtp_insecure is set to true, there is no such error. However, in this mode, TLS is susceptible to main-in-the-middle attacks.

Expected behaviour

Clients should be able to configure app.mailer.smtp_insecure to false, as this is the recommended setting when sending mails in production environments.

Solution

• Golang's tls module expects a ServerName if InsecureSkipVerify is set to false (Reference to source code). Without that, it raises the error: tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config.
• The ServerName is used to verify the hostname on the returned certificates.
• More details about ServerName and InsecureSkipVerify can be found in the tls documentation.
• This PR fixes the issue by setting ServerName in the tlsConfig, irrespective of the value of InsecureSkipVerify

[vercel]

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Name	Status	Preview	Updated (UTC)
frontier	✅ Ready (Inspect)	Visit Preview	Sep 14, 2023 9:57am

[coveralls]

Pull Request Test Coverage Report for Build 6183836917

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 51.28%

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Totals
Change from base Build 6168126313:	0.0%
Covered Lines:	6049
Relevant Lines:	11796

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💛 - Coveralls

[kushsharma]

LGTM