update the default email header image

[chris-okuda]

Scope of changes

Updates the default email header image to use the recently added static header image on rotational.io at https://rotational.io/img/apps/quarterdeck/rl_email_header.png. Also removed styles from the header image on the email template base.html because they were not necessary for this logo.

Type of change

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

Acceptance criteria

(The "welcome user" and "reset password" emails both use the same base.html with the logo header, so I included the easier "reset password" example below.)

Definition of Done

• I have manually tested the change running it locally (having rebuilt all containers) or via unit tests
• I have added unit and/or integration tests that cover my changes
• I have added new test fixtures as needed to support added tests
• I have updated the dependencies list if necessary (including updating yarn.lock and/or go.sum)
• 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 notified the reviewer via Shortcut or Slack that this is ready for review
• Documented service configuration changes or created related devops stories

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?

[daniellemaxwell]

Looks good!