Email builders

[pdeziel]

Scope of changes

This adds some builder utilities which create sendgrid emails from templates. The templates are loaded from a directory using go:embed to ensure that they exist at compile time.

The welcome email template is stub for right now to make sure the process is working.

Fixes SC-13278

Type of change

• new feature
• bug fix
• documentation
• testing
• technical debt
• other (describe)

Acceptance criteria

This story is mostly about making sure that the template strategy works and it unblocks the template creation stories.

Author checklist

• I have manually tested the change and/or added automation in the form of unit tests or integration tests
• I have updated the dependencies list
• I have recompiled and included new protocol buffers to reflect changes I made
• I have added new test fixtures as needed to support added tests
• Check this box if a reviewer can merge this pull request after approval (leave it unchecked if you want to do it yourself)
• I have moved the associated Shortcut story to "Ready for Review"

Reviewer(s) checklist

• Any new user-facing content that has been added for this PR has been QA'ed to ensure correct grammar, spelling, and understandability.
• Are there any TODOs in this PR that should be turned into stories?

[shortcut-integration]

This pull request has been linked to Shortcut Story #13278: Create Email Templates using Go Embed.

[bbengfort]

Very cool! go:embed is so much easier than go-bindata!