Support Twilio Email (SendGrid) Templates

[mfbrown]

Note: This ticket depends on #1975

Description

Our current email notifications are essentially only useful for testing; we need better templates to use for production use cases. We could write our own HTML templates for this, but we'd quickly end up wanting to provide a lot of configuration options for those templates which is a lot of work!

Meanwhile, both of our messaging providers support their own email editors to design & build transactional email templates:

• Mailgun: https://help.mailgun.com/hc/en-us/articles/6923289387675-Mailgun-s-Email-Editor#passport-start
• Twilio SendGrid: https://docs.sendgrid.com/glossary/transactional-email-templates

For example, our current emails look like this:

Meanwhile, Mailgun's editor looks like this (gives users a ton of control!):

...with the ability to insert template variables like:

Assumptions

• A single email template should be sufficient for all our emails at first
• Each email notification would have template variables for "subject" and "message"

Acceptance Criteria

• Add support for using Sendgrid templates for all email notifications (identity verification, request receipt, request review, request completion)
• Add ability to automatically populate Sendgrid with a default "Fides" template (if the user does not want to create their own)
• Add support for using Twilio templates for all email notifications (identity verification, request receipt, request review, request completion)
• Add ability to automatically populate Twilio with a default "Fides" template (if the user does not want to create their own)

Implementation Notes/Details

For an example of how the Mailgun API works:

curl -s --user 'api:YOUR_API_KEY' \
https://api.mailgun.net/v3/YOUR_DOMAIN_NAME/messages \
-F from='Sender Bob <sbob@YOUR_DOMAIN_NAME>' \
-F to='[email protected]' \
-F subject='Hello' \
-F template='template.test' \
-F t:variables='{"title": "API documentation", "body": "Sending messages with templates"}'

[rsilvery]

@TheAndrewJackson , @SteveDMurphy we'll discuss in refinement today - going to ask Steve to take this on because we need some data map work for Andrew

[Roger-Ethyca]

tested a while back but forgot to close. closing now