Add ability to use mailgun templates

[sanders41]

Closes #1865

Code Changes

• Checks if a fides mailgun template exists
• If the template exists mailguns emails are sent with the template, if not it falls back to the generic template.

Steps to Confirm

• Create a fides template in mailgun that contains a {{{fides_email_body}}} variable (I can share my api key with whoever tests this that already has this setup).
• Add the following variable to the docker-compose.yml

FIDES__EXECUTION__SUBJECT_IDENTITY_VERIFICATION_REQUIRED=true
FIDES__NOTIFICATIONS__NOTIFICATION_SERVICE_TYPE=mailgun
FIDES__NOTIFICATIONS__SEND_REQUEST_COMPLETION_NOTIFICATION=true
FIDES__NOTIFICATIONS__SEND_REQUEST_RECEIPT_NOTIFICATION=true
FIDES__NOTIFICATIONS__SEND_REQUEST_REVIEW_NOTIFICATION=true

• Create a mailgun messaging config.
• Create a privacy request and verify the identity verification email uses the mailgun template

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated:
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

Write some things here about the changes and any potential caveats

[sanders41]

@conceptualshark this will need some docs updates after it is reviewed. Do we have a process for that in the new docs repo yet? Can I just update docs there as I would have in this repo?

[conceptualshark]

@sanders41 I still have to update the issue/PR templates, but the new docs repo should work basically the same. There's a few quirks, but you should be able to make markdown edits, or open a ticket there for me to pick up!

[eastandwestwind]

Leaving a note that I've tested pointing to a mailgun acct with the fides template, and confirmed that the template is being used: