Add feature flag to remove the label "Menu" from the Menu button

What does this MR do and why?

Removes the "Menu" text from the primary menu dropdown when the new_navbar_layout feature flag is set.

This also adds a tooltip when hovering over the hamburger dropdown toggle.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console (rails c) enable the feature flag

   Feature.enable(:new_navbar_layout)

2. Visit any page where the top navbar is visible
3. Observe that the "Menu" text is absent and a tooltip is visible on hover

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Part of #366082 (closed)