Add a PNG email logo for HTML emails

Problem/Motivation

HTML emails on The Rightup sites take the logo from the default theme, which is logo.svg. Many email clients cannot show SVG images, including Outlook for Windows and Gmail webmail, so those readers see no logo.

The theme also ships logo.png, but it is 476×283, so an email would show it at several times the header size.

Steps to reproduce

1. Install Varbase 11.0.x-dev with The Rightup 1.0.x-dev.
2. Send any HTML email (for example a password reset) and open it in Outlook for Windows or Gmail webmail.
3. The header has no logo.

Proposed resolution

Add logo-email.png: logo.svg as a 123×73 PNG, the size the site header shows the logo at (7.675rem wide). The Easy Email Theme template sets no width on the logo, so a larger image would show at its full pixel size. The README says what the file is for. Same approach as Varbase Theme - Bootstrap 5 in #3637288.

The Rightup then points the email logo at it (see Related issues), so this should be released first.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ❌ Developer Documentation support
• ➖ User Guide Documentation support
• ❌ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Added logo-email.png, a PNG logo for HTML emails.

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