Add missing 90x avatar size for image scaling

What does this MR do?

While testing caching behavior for rescaled images, I noticed that the 90x90 avatar on the user profile page (the top-most header image) is never rescaled, but is delivered directly via send-url instead.

I found that this is because we're not including this size in our allowlist for avatar sizes. This MR fixes that.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done