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

[davide-negretti]

References

• Fixes Header and navbar refactoring #2656 for DSpace 7.x
• Fixes DOM contains duplicated HTML elements #2368 for DSpace 7.x
• Fixes [Accessibility] "All of DSpace" menu in header does not expose the presence of the submenu & its state via ARIA attributes #2699 for DSpace 7.x
• Fixes Accessibility of the administration menu (submenus) on the left hand side  #2178 for DSpace 7.x
• Porting of Header, navbar, and admin sidebar refactoring #2676 from DSpace 8

Checklist

• 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.
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[tdonohue]

👍 Thanks @davide-negretti ! I've verified this works for 7.x and it appears to apply all the same fixes as #2676 I've retested all the various accessibility fixes on 7.x and they all appear to have been fixed there as well.

I'm merging this into dspace-7_x. However, this PR likely requires mention in the 7.6.2 Release Notes because it refactors the header/navbar/side menu. Those refactors may cause some errors to occur in custom themes when sites upgrade from 7.6.1 to 7.6.2