Support ability to use email templates from Mailgun

[NevilleS]

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 Mailgun templates for all email notifications (identity verification, request receipt, request review, request completion)
• Add ability to automatically populate Mailgun 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"}'

[mfbrown]

Some questions we need to answer first:

• Does our current domain sending key allow us to use an existing template and template variables?
• Our domain sending key might not allow us to create a default template, can we just send the full default email HTML OR hard code a template name?

Other notes:

• If sending the email fails because of a template issue, then differentiate that error from other existing errors - so that we can tell the user how to correct the error.

[mfbrown]

Also - we should spend some time researching the Twilio/SendGrid API and ensure that we build this with as much commonality as possible.

#1866

[NevilleS]

Some responses...

Does our current domain sending key allow us to use an existing template and template variables?

We can test that out but I believe so. However, just to clarify an important point: if a user sets up their own Mailgun account and their own Fides instances, they can generate as many API keys as they like, and that should be the primary use case we plan for. Using a domain sending key is a special case where you want to use a single Mailgun account across multiple domains 👍

Our domain sending key might not allow us to create a default template, can we just send the full default email HTML OR hard code a template name?

We can't continue to send the "default email HTML", because it's not styled enough. However, we definitely could "hardcode" a default template name and provide docs that tell a user how to manually create their own template using the Mailgun UI. It looks like this, so it's pretty functional to start from scratch if they must: