7.x - Header, navbar, and admin sidebar refactoring

[davide-negretti]

References

• Fixes #2656 on DSpace 7.x
• Porting of #2676 from DSpace 8

Checklist

• [x] My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• [x] My PR passes ESLint validation using yarn lint
• [x] My PR doesn't introduce circular dependencies (verified via yarn check-circ-deps)
• [x] My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• [x] My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• [x] If my PR includes new libraries/dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• [x] If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself.
• [x] If my PR fixes an issue ticket, I've linked them together.