Allow users to set SMTP configuration on portal

[mr9dollars]

User can configure external SMTP provider thru the Portal, so the transactional emails will be sent with addresses of their domains.
After setting the provider and the default "from" address, user can send a test email from the Portal

• Design a SMTP provider setting page in the Portal [Design] SMTP provider setting page in the Portal #1353

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Users can choose from common providers or a custom one:

• Sendgrid Support Sendgrid as SMTP provider #1354
• Mailgun
• Maildrill
• Custom

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Documentations

• Docs: Guides for configuring the provider [Docs] Document how to configure custom email provider docs#51

[fungc-io]

@louischan-oursky

Summary of our offline discussion:

• After pressing the "Send Test Email" button, there will be an input dialog for the testing "To" address. The default value is the logged-in user's address. The dialog will dismiss after confirmation.
• The "Send Test Email" button should be disabled while loading.
• No need for a rate limit for testing.

Test email content
Subject: [Test] Authgear email
From: {{default from address from the config}}
Body: This email was successfully sent from {{tenent name}}

[fungc-io]

regarding #1452

• The on/off toggle should be disabled before re-auth
• From address should use default sender
• typo "SMPT" -> "SMTP" in the test email dialog box
• the help text below the provider selections switched place

[louischan-oursky]

Point 1, 3, 4 have been fixed and deployed. Point 2 turned out to be a configuration problem.

[fungc-io]

Replaced by #4217