Make dropdown menus on navigation to use full viewport width

What does this MR do?

Fixes alignment issues with navigation dropdowns and makes them consume full viewport width.

Screenshots

Projects Dropdown

More Dropdown

New Dropdown

User Dropdown

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#37349 (closed)