Part 2 - Top nav responsive view

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
• All MR Parts...
  • Decouple content-wrapper and content-wrapper-margin
    • Part 1 - Setup for responsive top nav redesign (depends on parent MR)
      • Part 2 - Top nav responsive view (depends on parent MR)

What does this MR do?

This MR replaces the placeholder in Part 1 with the actual functional responsive nav.

Screenshots (strongly suggested)

How to test?

You will need to enable the :combined_menu feature flag locally. Once this feature is enabled, you should be able to test out the new top nav responsive view on windows smaller than 570px.

Mobile nav closed	Mobile nav opened	Mobile nav + new dropdown	Mobile nav + new dropdown on project

Mobile nav > Projects	Mobile nav > Groups	Signed out + mobile nav closed	Signed out + mobile nav open

Does this MR meet the acceptance criteria?

Conformity

• [ ] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?) Not needed since this is behind a feature flag.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.

Related to #301143 (closed)