Frontend for top nav menu redesign

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
• All MR parts ...
  • Add keep_alive_slots component (merged)
    • Frontend for top nav menu redesign (depends on parent MR)

What does this MR do?

This MR implements the first part of the top nav redesign which combined the Projects, Groups, and More dropdown in the top nav bar. It does not do the 2nd part which implements the full-screen responsive redesign (this is coming next)...

These changes are behind the feature flag combined_menu.

How to test?

You'll need to enable the feature flag combined_menu. First start a rails console in the gitlab directory:

# Start a rails console in the `gitlab/` folder
rails c

Then enter:

Feature.enable(:combined_menu)

Screenshots (UPDATED 2021-05-14)

Feature flag off (nothing should be changed)

Projects dropdown	Groups dropdown	Projects dropdown responsive view

Feature flag on

PLEASE NOTE:

• The responsive view is not the final expected behavior. This is just Part 1 of the requirements. The full screen responsive part is coming up.

Projects explore page	Milestones page	Responsive (Projects)	Responsive (Groups)	Signed out

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 because changes are behind 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.

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.

References

Related to #301143 (closed)