Add support for Mailjet

[baus]

Got some code for us? Awesome 🎊!

Please include a description of your change & check your PR against this list, thanks!

• There's a clear use-case for this code change, explained below
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test:all and yarn lint)

We appreciate your contribution!

[github-actions]

It looks like this PR contains a migration 👀
Here's the checklist for reviewing migrations:

General requirements

• ⚠️ Tested on the staging database servers
• Satisfies idempotency requirement (both up() and down())
• Does not reference models
• Filename is in the correct format (and correctly ordered)
• Targets the next minor version
• All code paths have appropriate log messages
• Uses the correct utils
• Contains a minimal changeset
• Does not mix DDL/DML operations
• Tested in MySQL and SQLite

Schema changes

• Both schema change and related migration have been implemented
• For index changes: has been performance tested for large tables
• For new tables/columns: fields use the appropriate predefined field lengths
• For new tables/columns: field names follow the appropriate conventions
• Does not drop a non-alpha table outside of a major version

Data changes

• Mass updates/inserts are batched appropriately
• Does not loop over large tables/datasets
• Defends against missing or invalid data
• For settings updates: follows the appropriate guidelines