[Cleanup] Reduce duplication in code for Initial Custom Branding

[tmarkley]

In PR #826 I suggested creating a more generic custom logo component to solve the common logic used across the application for custom branding. We don't have enough time to implement and test that refactoring work ahead of v1.2.0, so this issue serves as the follow-up task. We maintainers will address these changes ahead of the v1.3.0 release.

User story:
"As a developer or plugin author, I'd like to easily reference branding asset (logo or mark) without caring or knowing about the branding configuration, validation, or fallback logic."

[kavilla]

Also the config should have a default value instead of /

[kavilla]

#941 (comment)

More granular error message