Expose theme and color scheme user preferences in API

What does this MR do?

Exposes theme_id and color_scheme_id in the admin users API to allow an admin to modify more user preferences through the API. This is useful for cases where multiple users need to be copied from one GitLab instance to another while retaining more of their user preferences.

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